Add an incomplete CSS serializer.

Serialization is considered correct if it maintains the invariant:

  parse(serialize(parse(text))) == parse(text)

for any text (Unicode string.)
This is not the case of this code yet (see #24),
but it’s good enough for error reporting in Servo.
De-duplicate identifier serializer logic.
Merge pull request #25 from SimonSapin/serializer

Add an incomplete CSS serializer.