remove unneeded extern keywords from function signatures on commoncrypto
Merge branch 'master' into common-crypto-base

* master:
  backend support check now lists which backend caused the skip
  add to author list
  Also use an absolute path for the about file
  Include a long description in our setup.py
  Explanatory comment
  So the tests don't all explode
  fixed typo
  flake8
  test for ctr
  Tests for OFB and CFB
  Implement this for CTR
  Validate the IV/nonce length for a given algorithm.
rework the namedtuple for hash methods
Merge branch 'master' into common-crypto-base

* master:
  improve docstring
  fix indentation mistake
  import build_ffi directly
  remove verify_kwargs and replace with pre_include/post_include/libraries
  refactor bindings to reduce code duplication with multiple backends
  instantiate hash objects for hmac checks too
  Instantiate our hash objects used for supported checks
commoncrypto binding now using new build_ffi method