Merge remote-tracking branch 'brett/itsdangerous'

Merge remote-tracking branch 'brett/itsdangerous'

* brett/itsdangerous:
  Call is_updated instead of testing it boolean.
  Harden It's Dangerous key management.
  First tests for the Session class.
  Set a starting value for session.send_new_cookie.
  Remove beaker stuff from the code.
  Delete the session cookie on an empty session.
  Back sessions with It's Dangerous.
  Improve fs security for itsdangerous secret.
  Docs for get_timed_signer_url.
  Basic itsdangerous infrastructure.

Conflicts:
	mediagoblin/tests/test_cache.py