Issue #24 Created PublicKey and PublicKeys interfaces.
Issue #24 Added RtPublicKey and RtPublicKeyTest.
Issue #24 Added RtPublicKeys, RtPublicKeysITCase, RtPublicKeysTest.
Issue #24 Added MkPublicKey, MkPublicKeys, MkPublicKeyTest,
MkPublicKeyTests classes. Added User.key() method.
Issue #24 Added @Immutable annotation to PublicKey and PublicKeys.
Merge pull request #188 from carlosmiranda/24

#188: pull request Issue #24 Implemented User.keys(), PublicKeys and PublicKey interfaces.