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
raise an exception if you try to use a HashContext after finalize is called.
Merge pull request #249 from dreid/hmac-context-from-backend

Get a HashContext from the hmac backend like we do a CipherContext
Remove unnecesary slice bounds
Call out the security reporting info in the CONTRIBUTING file
Merge pull request #252 from alex/silly-slice

Remove unnecesary slice bounds
Try this element
Another element
Donald says this is good
giv eup and use a header
Merge pull request #254 from alex/contributing

Call out the security reporting info in the CONTRIBUTING file
Remove the hash and hmac specific classes, now that the logic is the Context classes they don't really add value
Also remove teh ciphers specific class
Document AlreadyFinalized.
Explain ways in which we can make our docs stronger
Describe that hashes get real broken over time
Import AlreadyFinalized instead of exceptions.
Merge pull request #255 from alex/remove-backends

Remove the hash and hmac specific classes, now that the logic is the Con...