Fix #51: identifier instead of a quoted string in :contains()

Although `:contains()` only exists in [an early draft](http://www.w3.org/TR/2001/CR-css3-selectors-20011113/#content-selectors) of the Selectors spec, it is specified as taking argument that is either a quoted string or an identifier.

This is untested, but I fairly confident it is correct as [the same code](https://github.com/SimonSapin/cssselect/blob/1b95a44/cssselect/xpath.py#L414) is [tested in cssselect](https://github.com/SimonSapin/cssselect/blob/master/cssselect/tests.py#L354).
Update error message for :contains() change in the previous commit.
Merge pull request #52 from SimonSapin/patch-2

Fix #51: identifier instead of a quoted string in :contains()