(#7) Added several methods to the User model to gather some stats (yearly,
monthly, weekly).
Added two new utilities:
- timedelta_to_hms (so we can print timedelta objects properly in template
code)
- get_week_days (returns a list of datetime objects for the days in the
same week as a given day)
Added a template_helpers module, containing code that affects template
rendering.
Added timedelta_to_hms as a global to the default template rendering context
Refactored some code in the Workout model so it uses timedelta_to_hms instead
of running the same code twice.