Datashape

Python 3.6 compatibility for the test suite.

New Features
- Adds `datashape.discover` support for `types.MappingProxyType`
objects in python 3 and `types.DictProxyType` in python 2.
- Adds `datashape.discover` support for `collections.OrderedDict`
objects.

Bug Fixes
- Fixes `datashape.promote` to handle `datashape.string` types
correctly when mixing with `datashape.Option` types.

- Disallow duplicate fields in `Record` measures
- NumPy supports `NaT` in datetime and timedelta dtypes so support `Option` for them
- Type variable doc clarifications
- More thorough testing that repr produces an eval-able dshape