Fix XPath quoting in :contains()

The `xpath_literal()` method returns a valid, string-typed XPath expression for any string value, even on values that contains both `'` and `"` characters.

This change is untested, but see cssselect’s [test_unicode_escpapes](https://github.com/SimonSapin/cssselect/blob/1b95a44cd990abe3682b8c2ec250478d3673fd97/cssselect/tests.py#L427) for example of test cases.
Merge pull request #38 from SimonSapin/patch-1

Fix XPath quoting in :contains()