Font variations for custom loader w/tests & docs

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.
Merge branch 'master' into custom_variations
switched var around
Merge pull request #80 from dgreisen/custom_variations

Font variations for custom loader w/tests & docs