combine 30 day stats on explore page (closes #35)

* caffeine/models.py:
  - remove limit code from CaffeineManager.top_consumers_total and
    CaffeineManager.top_consumers_average
  - add new method top_consumers_recent
* caffeine/tests/test_models.py:
  - adapt tests to changed model code
* caffeine/views.py:
  - use CaffeineManager.top_consumers_recent
* caffeine/tests/test_views.py:
  - check that recent mate and coffee statistics are in the explore page's
    template context
* templates/explore.html:
  - combine the boxes for 30 day totals and averages