[lxml] [Pull 30] Use regexp for cssselect ":contains()" pseudo-class

This provides: * portability with other xslt processors * removes the 'css' global prefix assignment which can cause unexpected behaviour when working with xml files using that prefix * a slight performance improvement Users now need to explicitly pass in the regexp namespace when calling the ``xpath()`` method if they use the deprecated ``:contains()`` pseudo-class. https://github.com/lxml/lxml/pull/30 Laurence