Added sprints view

* Massive refactoring and API changes
* README update pending!
* Added get_sprints utility function (tests pending!)
* Added new mandatory RAPID_PROTOTYPING_SPRINTS_MODULE setting which should
  point to something like `myproject.context.sprints.sprints`
* Added new mandatory RAPID_PROTOTYPING_HOURLY_RATE setting
* Added many new columns for each task to support Scrum-like workflows
* Added sprints html and view
* Added templatetag get_difference to calculate difference between planned and
  actual time
* get_hourly_rate tag now uses the new setting mentioned above