Reentry

Changed
- don't resolve symlinks when hashing executable [67](https://github.com/aiidateam/reentry/issues/67)

Added
- add `data_file` key in configuration option
- add `REENTRY_DATA_FILE` environment variable

Changed
- drop support for `reentry_scan` hook since pip 19 introduced build-system isolation that restricts
scans performed by the hook to the build-system (containing only setuptools, wheel, reentry, etc.).
- `manager.scan`: renamed negated options (`nocommit` => `commit`, `nodelete` => `delete`)

Added
- add `reentry clear` command to clear the reentry cache

Changed
- replace py with pathlib/pathlib2, reducing dependencies for python 3
- config file:
- use hashed (shorter) filename
- resolve symlinks in file path
- recognize `XDG_CONFIG_HOME` environment variable
- move entry points, classifiers etc. to `setup.json` file

Changed
- data file is now composed of the directory in which the python executable sits and the python major version (2 or 3)
- setuptools relevant entry points can now be registered if specifically asked for
- `PluginManager.scan`: `group_re` kwarg now allows string regex too
- `PluginManager.iter_entry_points`: now scans by default if group or entry point not found
- `PluginManager`: new constructor kwarg `scan_for_not_found` defaults to `True`, set to `False` to fail faster when no cached group / entry point is found.
- `JsonBackend`, `BackendInterface`: The `write_*_dist()` methods have been replaced by `scan_*dist()`, the output of which can be passed to the `write_dist_map()` method.
- `JsonBackend.epmap`: promoted to read-only property, returns copy of internal map.

Changed
- data file name based on `sys.executable` to make sure entry points registered during install phase are available afterwards
- `reentry_scan` during install no longer overwrites, but only adds (distributions installed simultaneously in one `pip` invocation can not discover each other's entry points and might overwrite each other)

Added
- setup coveralls
- added coverage badge to REAME
- cli: `reentry dev coveralls` runs coveralls only if TRAVIS env variable is set
- read data dir from REENTRY_DATADIR env variable (env > rcfile > default)
- CI test for registering entry points from the plugin host
- documented limitations of `reentry_scan`
- documented compatibility issues with `setup_requires`