Initial work on a prioritized multibackend for which implements CipherBackend

A large impetus for this is that without this, for most OS X users, cryptography is a regression from PyCrypto in terms of being able to use CTR mode.

Needed:

- [x] docs
- [x] are we testing this the right way
- [x] probably coverage
- [x] needs to handle when one it's constituents doesn't implement an interface