Added support for rasterizing geometries (issue 45) and associated test cases.
Added example script for rasterize.
Cleaned up comments and docstring.
Merge branch 'master' of https://github.com/brendan-ward/rasterio into brendan-ward-master
Modifications to pull request #62.

I made rasterize_features redundant. With a single list comprehension
a list of features can be turned into a list of (geom, value) pairs.

I made the default value 255 to match other rasterio functions.

_rasterize() now takes responsibility for converting Python geoms to
JSON and can now burn shapes into an existing image array.