[WIP] PHPCR Browser

I've restored this PR to life and am battling with the same problem you had with the forms.

The direction I'm going in with the Form stuff is probably 50% wrong. I thinkusing a Value Transformer might be a better idea.

Have renamed a few of the things to be  more indicative of PHPCR and I inject the Sonata AdminPool so that we extend the correct admin template, as noted this couples the controller to Sonata, but I don't know of a better way to do this.

I'm guessing this would be better as a direct PR to master, but in anycase its good to compare against yours.