Add support for `only` in the AST

[tilomitra]

We want to be able to check to see if a media query has the only keyword. IE8 can use it to ignore certain MQs.

[tilomitra]

Submitted a PR for this. #4

[tilomitra]

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

[ericf]

According to the spec:

The keyword ‘only’ can also be used to hide style sheets from older user agents. User agents must process media queries starting with ‘only’ as if the ‘only’ keyword was not present.

http://www.w3.org/TR/css3-mediaqueries/#syntax