Added repository initializer

This is the method provided by the DoctrinePHPCRBundle to both register node types and create the initial repository structure.

It would therefore mean we could remove both `sulu:phpcr:init` and `sulu:webspace:init` commands.

It should however be complemented with some minimal fixtures for the homepage.

__TODO__:

- [ ] Register node types (as CND files or we could use the auto register feature in the new PHPCRODM Bundle)
- [ ] Homepage / route fixtures
Having these as separate parameters sucks, I think there should be some configuration context, or we could even use constants (as recommended in the Symfony BP book :P)