Added a bloom filter to CSS selector matching.

Every other browser engine uses a bloom filter to quickly reject `Descendant`
selectors. This adds that feature to servo, and it even works (mostly!) in
parallel!
Here's what Gecko uses (it's an `nsIAtom`, which is the type used by class names/tag names/IDs): http://mxr.mozilla.org/mozilla-central/source/layout/style/nsRuleProcessorData.h#72

Looks like Gecko uses element tags and classes: http://mxr.mozilla.org/mozilla-central/source/layout/style/nsCSSRuleProcessor.cpp#3636