Fix rev (cache busting) support

* Update to Yeoman-maintained `grunt-filerev`
* Add JS support (via [#235][])
* Add relative URLs to leaflet.fullscreen

Closes item:52

[#235]: https://github.com/yeoman/grunt-usemin/issues/235#issuecomment-33316221
Merge pull request #126 from tlvince/bug/rev

Fix rev (cache busting) support