Mapping to PHPCR (alternative to #95)

This is the same as #95 but mapping to `Doctrine\Phpcr`.

I am doing this because as I refactored I became less confident that this is the best way.

Perhaps the best way to approach this would be to actually implement ORM support and see how it fits in - this might also help adoption outside of the CMF community. But that would be post 1.0.

Anyway, quite undecided about this now. We can either

- Keep everything in /Model and risk BC break in the future.
- Put concrete implementations in `Doctrine\Phpcr` and make the Bundle work with that.
- Do as above but refactor the bundle to properly support multiple backends.

I guess its a gamble about if we will need #2 in the future.