Replaced the regex-based transformation of CSS to XPath

The library now uses the Symfony CssSelector component, which supports
more cases than the custom logic used previously.
Closes #34
Preserver mediaqueries

This searches all style tags for Media Queries, and replaces the content with just those. Fixes #45

Also set the default to strip mediaqueries from inline tags, because that doesn't work as expected. (at least, not after #52)

Not sure if this is the most performant/optimal way, so open for ideas ;)
Increase base for specifity

Following http://www.w3.org/TR/selectors/#specificity, the specifity is concatenating a,b and c. It notes that the base should be large enough. So it should be something like 2,9,3= 293, but 2,11,3 (=313) should be lower then 3,0,0 (300). This make 2,11,3 = 21103 and 3,0,0 = 30000. So it is still not always perfect, but better ;)
Merge remote-tracking branch 'origin/patch-2'
Merge remote-tracking branch 'origin/use_symfony_css_selector'