Formulaic

This is a minor patch release that brings in some valuable improvements.

- Keep track of the pandas dataframe index if outputting a pandas `DataFrame`. (issue noted by CamDavidsonPilon)
- Fix using functions in formulae that are nested within a module or class. (issue noted by bashtage)
- Avoid crashing when an attempt is made to generate an empty model matrix. (PR from bashtage)
- Enriched setup.py with long description for a better experience on PyPI. (PR from bashtage)

This is major release that brings in a large number of improvements. The API is still not finalised (that will happen with the 1.0) release, and so be careful of API breakages brought in by subsequent major releases (e.g. v0.3.x). Many thanks to those who have contributed by testing or code submissions (including bashtage, griffiri, and CamDavidsonPilon).

**New Features and Improvements:**
- Enriched formula parser to support quoting, and evaluation of formulas involving fields with invalid Python names.
- Added commonly used stateful transformations (identity, center, scale, bs)
- Improved the helpfulness of error messages reported by the formula parser.
- Added support for basic calculus on formulas (useful when taking the gradient of linear models).
- Made it easier to extend Formulaic with additional materializers.
- Many internal improvements to code quality and reliability, including 100% test coverage.
- Added benchmarks for Formulaic against R and patsy.
- Added documentation.
- Miscellaneous other bugfixes and cleanups.