Explicit backend
Add a default_backend and start updating docs.
Fix doctests.
Use backend as keyword argument everywhere.
Document all the parameters including cross references to specific providers where appropriate.
Test that openssl is the default backend.
Use keyword argument forms everywhere.
Put some docs for default_backend in a place.
Use backend as keyword even when copying hmacs.
Fix pep8
Merge pull request #227 from dreid/explicit-backend-in-hazmat

Explicit backend