Search filters now apply to UTF grid as well as tiles

Previously we had an OpenLayers subclass of "Layer.XYZ" with the filtering logic.
I couldn't see how to make a similar subclass of "Layer.UTFGrid" without duplicating code,
so instead we apply the new function makeFilterableLayer to both XYZ and UTFGrid layers.
It is conventional in node-style modules to drop the '.js' in require statements.
Merge pull request #125 from RickMohr/filter-utf-grid

Search filters now apply to UTF grid as well as tiles