add cipher support to the commoncrypto backend
update changelog for commoncrypto backend
add error handler
add comment and change gc for cipher context
change cipher registry to store enums
make register_cipher_adapter private and fix cryptorref release
Merge branch 'master' into commoncrypto-cipher-backend

* master:
  expand tox backend example
  On OS X at build time compile the CC bindings
  fix docs
  update docs for name attribute
  revert fixture decorator for now, switch to append. no more globals
  docs for explicit backend selection and document name attribute of backend
  modify backend selection to allow multiple backends via comma delimiter
  better name for the variable
  don't mutate _ALL_BACKENDS
  pass posargs via tox so --backend can be used for tox envs
  support --backend as a pytest flag to limit to one backend for testing
style nit fix
missed one. of course.
Merge pull request #486 from reaperhulk/commoncrypto-cipher-backend

Add CommonCrypto cipher backend