* Created NoiseBackend class serving as a base for backends
* Refactored NoiseProtocol name parsing
* Refactored existing spec-defined functions into abstract classes.
Implementing classes are connecting crypto primitives to expected
interfaces.
* Refactored existing usage of Cryptography as source of crypto into
"default" backend (along with in-house implementation of X448).
* Provisioned "experimental" backend, it will contain e.g. non-default
crypto algorithms
* Backend can be chosen while creating NoiseConnection, though by
default, the Cryptography backend ("default") is used
Closes#7
* Allowed '/' in protocol name
* Added SymmetricState.GetHandshakeHash()
* Added CipherState.SetNonce()
* Unittests to ensure that each of above is working
Closes#3
