Add pbf as one of the options for tile formats.
add reference to mbliberator
Merge pull request #55 from calvinmetcalf/patch-1

add reference to mbliberator
Merge pull request #52 from mapbox/pbf-tiles

Add pbf as one of the options for tile formats.
webp file type added
revert extension to fix test
Merge pull request #56 from jaakla/master

webp file type added
Remove JSONP wrapper on grids