Begin work on features module for #20.
Working polygonize function (#20).

TODO: tests, better examples, removal of unneeded code copied
from Fiona.
Add test of polygonizing for #20.

No disk files used in this test :)
rasterio.features.shapes() yields feature shapes (#20).

GeoJSON-ish features are excessive, so now shapes() simply yields
a geom, image_value pair. This let me delete a bunch of Cython code.
It's easy enough to make features out of these if needed.
Add affine transform arg to shapes() (#20).

This gets handed to the in-memory vector feature store so that we
can get shapes in world coordinates.