[Fixes #3] Support only keyword

[tilomitra]

I added support for the only keyword in the AST, and allowed for it to be matched based on the only property in the values object. Here's an example:

mediaquery.match('only screen and (color)', {only: true, type: 'screen', color: 1}); //=> true
mediaquery.match('only screen and (color)', {type: 'screen', color: 1}); //=> false

I also added unit tests for this to maintain 100% coverage.

[tilomitra]

After discussions with @ericf, we've decided not to support only since it's basically a noop according to the spec.