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
I don't think the lack of type checking is a major concern, but I'm wondering if there is a different way to handle this (instead of a public method that returns an instance of a private constructor - and yes, I know that is how the `ol.source.TileDebug` does it as well).