issue2550949: Rate limit password guesses/login attempts. Generic rate limit mechanism added. Deployed for web page logins. Default is 3 login attempts/minute for a user. After which one login attempt every 20 seconds can be done. Uses gcra algorithm so all I need to store is a username and timestamp in the one time key database. This does mean I don't have a list of all failed login attempts as part of the rate limiter. Set up config setting as well so admin can tune the rate. Maybe 1 every 10 seconds is ok at a site with poor typists who need 6 attempts to get the password right 8-). The gcra method can also be used to limit the rest and xmlrpc interfaces if needed. The mechanism I added also supplies a status method that calculates the expected values for http headers returned as part of rate limiting. Also tests added to test all code paths I hope.

Docs
====

.. toctree::
   :maxdepth: 2

   docs/features
   docs/installation
   docs/upgrading
   docs/FAQ
   docs/user_guide
   docs/customizing
   docs/admin_guide
   docs/xmlrpc
   Design (original) <docs/design>
   docs/developers
   docs/tracker_templates
   docs/acknowledgements
   docs/license