Font variations for custom loader w/tests & docs

I removed the teststring code that you didn't appear to like in https://github.com/typekit/webfontloader/pull/51. I also added tests and documentation.

Added font variation support for the custom loader. variations are
appended to the family name using the FVD notation like so:

    WebFont.load({
      custom: {
        families: ['My Font', 'My Other Font:n4,i4,n7'],
        urls: ['/fonts.css']
      }
    });

Includes tests and update to the documentation.

Based on code by https://github.com/jessegrosjean.