Update GroovyExtensionMetaClassRegistry.groovy

Extend node's get metaclass extension so that it supports retrieval of referenced nodes.

The method used to retrieve nodes by UUID from session is taken from Jackrabbit's code: http://svn.apache.org/repos/asf/jackrabbit/trunk/jackrabbit-core/src/main/java/org/apache/jackrabbit/core/PropertyImpl.java

Currently, the Property API does not provide an easy way of retrieving referenced nodes if the property is multivalued.