INT-3140: Add `#xpath()` SpEL Function Support

JIRA: https://jira.springsource.org/browse/INT-3140

* Introduce `XPathUtils`
* Add `#xpath()` tests
* Add documentation

INT-3140: Polishing according PR comments

INT-3140 Polishing

- Doc and javadoc polishing
- Fix some compiler warnings (not all xpath)