Add mock implementation of nodes and properties

In unit tests its often enough to have a simple, static tree of JCR nodes whose properties can be changed instead of using a full-blown repository. Using such a mocked node structure instead of a RepositoryTestCase will have several advantages:

• only setting up a node structure will be significantly faster that starting a test repository for each test
• it will prevent the warnings output by the RepositoryTestCase
• it will be easier to create different fixtures for different tests