Dynamic resolver for reverse registrar

[jefflau]

ens/contracts/ReverseRegistrar.sol

Line 23 in 9f0dc10

defaultResolver = resolverAddr;

Testing on ropsten right now for reverse records and the setName is using an old public resolver. Seems like it would make sense to make this dynamic to get the address of 'resolver.eth' so as the public resolver gets upgraded, this contract will also use the updated resolver.

[Arachnid]

Does it matter that it uses a different resolver? The idea of setName is to provide a quick and easy option for just setting the name; it doesn't require broader support than that.