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
You'll need to also include documentation blocks for each of the option members (see other source option properties [for examples](https://github.com/klokantech/ol3/blob/ef816d30ed9163eac93761abbf2307c386e3c9bc/externs/olx.js#L3235-L3248)).