OGC Simple Geometries in Vector Tiles

[flippmoke]

Added ability to use strictly simple in mapnik vector tiles, bumped to version 0.11.0 of mapnik-vector tiles. Added tests to see if geometry is_simple and is_valid using boost geometry

[springmeyer]

Two thoughts:

• Ideally use tile_datasource_pbf for lower memory access to data
• What about instead of a boolean the function returning stats on # of valid/invalid geometries per layer? That seems a whole lot more useful than just returning early on one invalid geometry.

[flippmoke]

@springmeyer I could return invalid geometries but this was an easy first test, I was more concerned on just identifying problem tiles.

[springmeyer]

Would also be great to have explanations in the docs for what "simple" and "valid" mean exactly according to boost::geometry.