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 think we should stop using this (cargo culted) "alert alert-success" class.  This example demonstrates some great new functionality, and I think it would be great to make the presentation a bit less clunky.

I think showing the flag and country name in situ (or near the pointer) would be nice.  But I'm also a [bit biased](http://dev.openlayers.org/examples/utfgrid-geography-class.html).