Verify signature on encrypted response assertions.

Also, CryptoBackendXmlSec1.validate_signature() raises SignatureError so we need to handle that.
Signature verification raises MissingKey (SigverError) if only_use_keys_in_metadata config option is set and we have no certs for issuer.
Merge pull request #55 from rectalogic/encrypted-assertions

Support signature verification on encrypted response assertions