Add UTFGrid interactivity.

This required forking the angular leaflet directive library and adding
UTFGrid interactivity to it.
The docs for angular-leaflet-directive are non-extant.  I had to figure this stuff out by going to their examples and showing the source, or hunting through the source.  I'd thought it might be helpful to have some commented-out notes in at least one commit here, so we could find this stuff again later.