Use specificity module

The previous method for calculating specificity failed on pseudo classes. For example the specificity of `.editor:not(.mini)` was -1 (it should be > 0). I also updated the specificity score to match [the css3 specification](http://www.w3.org/TR/css3-selectors/#specificity)

# Previous method
specificity(".editor")
specificity(".editor:not(.mini)")

# output (took 3ms)
1000
-1

# New
specificity(".editor")
specificity(".editor:not(.mini)")

# output (took 2ms)
10
20