* add verifier and test using simulated backend
* add ContractCaller
* commit simulated backend after deploy and after smart contract writes
* use bind.NewKeyedTransactor for all transactions in tests
* rename RegistryVerifier to Verifier
* initialize contract verifier if MailServerRegistryAddress config is set
* use contractAddress.Hash()
* refactoring
* use fmt.Sprintf to format contract address in logs
* fix test and lint warnings
* update Gopkg.lock
* update Gopkg.lock once more
