Param

Minor release:

- Added positional default arguments for nearly all Parameter subclasses (apart from ClassSelector)
- Minor bugfixes for watching callbacks

For more details, you can see a [full list of changes since the previous release](https://github.com/holoviz/param/compare/v1.8.1...v1.8.0).

Major new feature set: comprehensive support for events, watching, callbacks, and dependencies

- Parameterized methods can now declare `depends(p,q)` to indicate that they depend on parameters `p` and `q` (defaulting to all parameters)
- Parameterized methods can depend on subobjects with `depends(p.param,q.param.r)`, where `p.param` indicates dependencies on all parameters of `p` and `q.param.r` indicates a dependency on parameter `r` of `q`.
- Functions and methods can `watch` parameter values, re-running when those values change or when an explicit trigger is issued, and can unwatch them later if needed.
- Multiple events can be batched to trigger callbacks only once for a coordinated set of changes

Other new features:

- Added support in ObjectSelector for selecting lists and dicts ([268](https://github.com/holoviz/param/pull/268))
- Added pandas DataFrame and Series parameter types ([285](https://github.com/holoviz/param/pull/285))
- Added support for regular expression validation to String Parameter ([241](https://github.com/holoviz/param/pull/241), [#245](https://github.com/holoviz/param/pull/245))

For more details, you can see a [full list of changes since the previous release](https://github.com/holoviz/param/compare/v1.8.0...v1.7.0).

Since the previous release (1.6.1), there should be no changes that affect existing code, only additions:

* A new param namespace object, which in future will allow subclasses of Parameterized to have much cleaner namespaces ([230](https://github.com/holoviz/param/pull/230)).
* Started testing on python 3.7-dev ([223](https://github.com/holoviz/param/pull/223)).
* param.version now provides functions to simplify dependants' setup.py/setup.cfg files (see https://github.com/holoviz-dev/autover/pull/49).

Although param should still work on python 3.3, we are no longer testing against it (unsupported by our test environment; [234](https://github.com/holoviz/param/pull/234)).

For more details, you can see a [full list of changes since the previous release](https://github.com/holoviz/param/compare/v1.6.1...v1.7.0).

Restores support for the previous versioning system (pre 1.6; see [225](https://github.com/holoviz/param/pull/225)), and fixes a number of issues with the new versioning system:

* Allow package name to differ from repository name (https://github.com/holoviz-dev/autover/pull/39)
* Allow develop install to work when repository is dirty (https://github.com/holoviz-dev/autover/pull/41)
* Fixed failure to report dirty when commit count is 0 (https://github.com/holoviz-dev/autover/pull/44)

Notable changes, fixes, and additions since the previous release (1.5.1) are listed below. You can also see a [full list of changes since the previous release](https://github.com/holoviz/param/compare/v1.5.1...v1.6.0).

Changes:
* `param.__version__` is now a string
* `param.version.Version` now supports a tag-based versioning workflow; if using the `Version` class, you will need to update your workflow (see [autover](https://github.com/holoviz-dev/autover) for more details).
* Dropped support for python 2.6 ([175](https://github.com/holoviz/param/pull/175)).
* No longer attempt to cythonize param during installation via pip ([166](https://github.com/holoviz/param/pull/166), [#194](https://github.com/holoviz/param/pull/194)).

Fixes:
* Allow `get_param_values()` to work on class ([162](https://github.com/holoviz/param/pull/162)).
* Fixed incorrect default value for `param.Integer` ([151](https://github.com/holoviz/param/pull/151)).
* Allow a `String` to be `None` if its default is `None` ([104](https://github.com/holoviz/param/pull/104)).
* Fixed `ListSelector.compute_default()` ([212](https://github.com/holoviz/param/pull/212)).
* Fixed checks for `None` in various `Parameter` subclasses ([208](https://github.com/holoviz/param/pull/208)); fixes problems for subclasses of `Parameterized` that define a custom `__nonzero__` or `__len__`.

Additions:
* Added `DateRange` parameter.

Miscellaneous:
* No longer tested on python 3.2 (unsupported by our test environment; [218](https://github.com/holoviz/param/pull/218)).

* Fixed error messages for ClassSelector with tuple of classes
* Added get and contains methods for ParamOverrides

A full list of changes since the previous release is available [here](https://github.com/holoviz/param/compare/v1.5.0...v1.5.1).