Raster pbf

Supports new raster embedding in `mapnik-vector-tile`.

- `Backend` source can now wrap a standard png/jpeg tilelive source instead of just vector pbf sources.
- Adds [tiletype](https://github.com/mapbox/tiletype) dependency for detecting tile format
- Drops `deflate` option from both Vector and Backend sources (this was never used anywhere)
- Adds raster support to xray style

@springmeyer seeing some test fixture differences on node v0.8.x, any discrepancies I should be aware of?