Implement serialize method on JaxbAssertions

The JaxbAssertion class inherits it's serialize method from PrimitiveAssertion which unhelpfully returns nothing.

This patch implements the serialize method using a JAXB marshaller. It uses similar code to JaxbAssertionBuilder to get a JAXBContext. It sets the "jaxb.fragment" flag of the marshaller so that the assertions can be included in larger xml documents (this is supported by all compliant implementations, see http://java.sun.com/javase/6/docs/api/javax/xml/bind/Marshaller.html).

The patch builds cleanly against both 2.2.2 and the current trunk.

Please include "from University of Southampton IT Innovation Centre" in the change log.