Add support for decoding existing tiles

Also adds a utility to convert tiles to GeoJson.

Since the last pull request i have added tests, some from https://github.com/mapbox/mapnik-vector-tile/blob/master/test/python/test.py, and some new ones.

I also changed VectorTile.to_geojson() from returning a string, to a dictionary, this is an incompatible change, but I'm guessing no one is actually using this code yet, so it is not a problem.

There is not yet complete test coverage for deserializing existing tiles, all of the tests so far deal with point features.

I'm planing on adding support for adding line and polygon features soon, and plan on adding more tests then.