Added 'processExternalEntities' to JAXB2Marshaller

Added 'processExternalEntities' property to the JAXB2Marshaller, which
indicates whether external XML entities are processed when
unmarshalling.

Default is false, meaning that external entities are not resolved.
Processing of external entities will only be enabled/disabled when the
Source} passed to #unmarshal(Source) is a SAXSource or StreamSource. It
has no effect for DOMSource or StAXSource instances.