Start adapting to mapnik 'image_data_any' branch

 - all tests should still pass against Mapnik 2.3.x
 - for now skips any image types besides rgba8
 - next step: add tests and support for int8, int16, and float32
Merge pull request #72 from mapbox/image_data_any

Start adapting to mapnik 'image_data_any' branch