Xpath escaping

Implemented xpath escaping.
Most likely this still won't cover all edge cases, but it does pass the tests I wrote for it (https://github.com/phpcr/phpcr-api-tests/pull/112).

If this would be ready to merge, #132 can be closed as the change for that issue is also made in this PR.

I don't really like the look of the escape method at this point, because of the assumption that we need to escape single quotes there. But at this point I think it's safe to make that assumption.

This PR should fix issues with quotes and backslashes (as #129 partly did)