Aura: Custom Verification logic

[rphmeier]

The AuraVerifier should be generic over some trait CustomVerifier (name subject to change).

Something similar to

trait CustomVerifier {
     // future that resolves when a block is verified or rejected. error type does not have to be `()`.
     type Verified: IntoFuture<Item=(),Error=()>;

     // verify a block under custom rules.
     fn verify(&self, block: &Block) -> Self::Verified;
}

The verification queue instance for now should just .wait() on the future returned.
A NullVerifier or other no-op implementation would be helpful.