merge changes to about page from PHP version
implement overall page

- caffeine.models:
  - move result dictionary creation from
    CaffeineManager.total_caffeine_for_user to total_result_dict
  - add methods to CaffeineManager:
    - total_caffeine
    - hourly_caffeine
    - daily_caffeine
    - monthly_caffeine_overall
    - hourly_caffeine_overall
    - weekdaily_caffeine_overall
- caffeine.views:
  - implement OverallView.get_context_data
- add templates/overall.html to display charts