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
These would be the first single-line conditionals in the library.  Not a big deal, but I'd suggest dedicated blocks for consistency.  We should really bring in `jscs` to [enforce this sort of thing](https://github.com/jscs-dev/node-jscs#requirecurlybraces).