Move register so it can be used by cryptography.hazmat.bindings.interfaces.
flesh out method definitions.
Change interface names to fit in the new smaller interface surface and
correct method names on interfaces.
Start documenting the backend interfaces.
Fix usages of interfaces.register
Document backend interfaces.
Hashes and HMACs.
Move more uses of interfaces.register...
Merge branch 'master' into backend-interfaces
Merge branch 'master' into backend-interfaces
Some docstrings.
Describe what backends provide via these interfaces and that not all backends must provide all interfaces.
Merge pull request #269 from dreid/backend-interfaces

Backend interfaces