Gtfstk

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- Updated the installation requirements in ``setup.py``.

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- Fixed the bug where ``setup.py`` could not find the license file.

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- Finally knuckled down and wrote a GTFS validator: ``validators.py``. It's basic, easy to read, and, thanks to Pandas, fast. It checks `this 31 MB Southeast Queensland feed <http://transitfeeds.com/p/translink/21/20170310>`_ in 22 seconds on my 2.8-GHz-processor-16-GB-memory computer. With the same computer and feed and in fast mode (``--memory_db``), `Google's GTFS validator <https://github.com/google/transitfeed>`_ takes 420 seconds. That's about 19 times slower. Part of the latter validator's slowness is its many checks beyond the GTFS, such as checks for too fast travel between every pair of stop times.
- Moved all but the most basic ``Feed`` methods into other modules grouped by theme, ``routes.py``, ``stops.py``, etc. Eases reading and additionally exposes the methods as functions on feeds, like in the GTFSTK versions before 7.0.0.
- Speeded up ``asssess_quality``.
- Refactored ``constants.py``.
- Renamed some functions.

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- Rewrote most feed functions as ``Feed`` methods.
- Rewrote tests for pytest.
- Removed some miscellaneous functions, such as plotting functions.

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- Changed ``feed.read_gtfs`` to unzip to temporary directory.
- Enabled ``feed.write_gtfs`` to write to a directory.

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- Improved function names, e.g. ``compute_trips_stats`` -> ``compute_trip_stats``.
- Added functions to ``cleaner.py`` and changed cleaning function outputs to feed instances.
- Made ``feed.copy`` a method.
- Simplified Feed objects and added auto-updates to secondary attributes.
- Changed the signatures of a few functions, e.g. ``calculator.append_dist_to_shapes`` now returns a feed instead of a shapes data frame.
- Fixed formatting of properties field in ``calculator.trip_to_geojson`` and ``calculator.route_to_geojson``.