UTFGrid Support

Implementation of the UTFGrid interaction according https://github.com/mapbox/utfgrid-spec

Done as `ol.source.TileUTFGrid` - as discussed in the ticket #922

A complete example of use is included - with displaying MBTiles tiles via TileJSON with the interactivity - showing flags for countries.
It is based on: https://www.mapbox.com/demo/visiblemap/

There is a possibility to use Mustache templates typically generated by TileMill.

Made by @petrsloup and contributed by Klokan Technologies GmbH.

Closes #922
So you propose adding `minZoom`, `maxZoom` constructor options that could be used to further clamp the zoom range specified in the json ?

Note: this part is identical to the `ol.source.TileJSON` (there are no options for this as well).