Add extension capabilities to the JCR-Mock framework

[danvalencia]

An idea would be to create a NodeFactoryListener interface, with methods such as nodeCreated and propertyCreated, and add the ability to register listeners to the NodeFactory. This would allow for a non-invasive way to extend JCR-Mock's functionality, e.g. to create a Mock libraries for Sling or CQ5 APIs on top of JCR-Mock.