Get a HashContext from the hmac backend like we do a CipherContext
Don't use self.__class__
Merge branch 'master' into hmac-context-from-backend

Conflicts:
	cryptography/hazmat/bindings/openssl/backend.py
	cryptography/hazmat/primitives/hmac.py
Merge pull request #249 from dreid/hmac-context-from-backend

Get a HashContext from the hmac backend like we do a CipherContext