Check for bad SECRET_KEY
Some fixes
Fixes after code review
Documentation for `check_secret_key`
Merge remote-tracking branch 'cool-RR/master'

* cool-RR/master:
  Documentation for `check_secret_key`
  Fixes after code review
  Some fixes
  Check for bad SECRET_KEY
Update changelog and authors file for SECRET_KEY check; thanks Ram Rachum.
Update tox.ini.