Get a HashContext from the backend like we do a CipherContext
Reflow Hashes.supported.
GCM loader support
Merge pull request #182 from reaperhulk/gcm-loaders

GCM test vector loaders + tests
Merge pull request #248 from dreid/hash-context-from-backend

Get a HashContext from the backend like we do a CipherContext
Drop random support for weird inheritance
Don't query OpenSSL for block sizes, we already know them
Merge pull request #250 from alex/drop-inheritance

Drop random support for weird inheritance
Merge pull request #251 from alex/dont-query-openssl

Don't query OpenSSL for block sizes, we already know them
Merge branch 'master' into hmac-context-from-backend

Conflicts:
	cryptography/hazmat/bindings/openssl/backend.py
	cryptography/hazmat/primitives/hmac.py