Initial work on a prioritized multibackend for which implements CipherBackend
HashBackend is now supported by PrioritizedMultiBackend
Added support for HMAC backend
Added PBKDF2HMAC support
Replace re-running all tests with some unit tests
Direct tests for the cipher support
Direct tests for the hash support
Only make teh default once
Not true anymore
More direct tests
Fix
pep8 fixes
hkdf
Refactor HKDF support and provide vectors for tests.
This got split into SHA1 and SHA256
Break up hkdf_derive into hkdf_extract and hkdf_expand.

Testing each individually against all the vectors and actually
asserting about the intermediate state.

hkdf_derive is now just a helper function which copes with the
default arguments.
Fix pep8.
Fix case.
Use the nist vector loader.
Closer to proposed interface in #513.