Create Modernizr build using Grunt-Modernizr.

## Changes
 -  Builds Modernizr using [Grunt-Modernizr](https://github.com/Modernizr/grunt-modernizr) rather than requiring a manual build. This lets us create specific Modernizr builds targeted at each client application, rather than having to include all tests here. Also less third-party code in repo! :tada: 
 - Renames the `.video` class because it was conflicting with Modernizr's video feature tests.