Add permission interface and lower dependency to 2.2

@manuakasam

This PR brings the PermissionInterface planed to ZF 2.3. I asked mwop and ZF 2.3 is not planned before Q1 2014, which is quite vague and I'd like to release sooner.

So basically, once we ship 2.3, PermissionInterface in ZfcRbac will extend built-in interface and be deprecated. The ZfcRbac PermissionInterfae will be removed for ZF 3.

I need to update the doc a bit, then I'll release a first beta.