Custom build

Allow users to customize their modernizr build. Instead of loading a modernizr.js file containing all tests, they now can load only a few of them, decreasing the amount of javascript to pre-compile.

This build also contain the modernizr.load() method, allowing users to use the gem as is, and a script to update all tests from the source with no intervention from the user.

This is an adaptation of the work I did for another gem wrapper for modernizr. This could be a fix for #23.