PHPCR wrapper

The PHPCR wrapper ultimately aims to be able to wrap all of the PHPCR
objects as required.

Currently it wraps the `Session`, `Node` and `Property` classes, and enables us to use our own classes and effectively
extend the nodes returned from the PHPCR repository.

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [ ] create documentation

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | #173 
| BC Breaks     | NO
| Doc           | not yet
Hmm. Is it better to put this here or in docs?