Rename feature types and properties

* changed feature types to `Point`, `LineString`, `Polygon` to match GeoJSON and VectorTile specs https://github.com/mapbox/mapbox-gl-style-spec/issues/52
* `VectorTileFeature.mapping` is now `VectorTileFeature.types` and is array, not object.
* renamed public `feature` properties `_type` and `_extent` to `type` and `extent`
* tidied up the code so that it's a bit cleaner