Experimental.cssselect

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* Fix bug with ignorable spaces between css selector groups. This caused
extremely slow Diazo execution times.

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* Make ``:contains()`` case sensitive for simplicity and speed.

* Update to lxml master to fix problems with ignorable spaces in selectors.

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* In lxml.cssselect, use the xpath 'A//B' (short for
'A/descendant-or-self::node()/B') instead of 'A/descendant::B' for the css
descendant selector ('A B'). This makes a few edge cases to be consistent
with the selector behavior in WebKit and Firefox, and makes more css
expressions valid location paths (for use in xsl:template match).

* Fork from lxml 2.3