add caffeine_api_v1

- caffeine.models:
  - add random_users method to CaffeineUserManager
  - implement __unicode__ method of User
- add app caffeine_api_v1 and implemente views.random_users
- coffeestats.settings.base:
  - add caffeine_api_v1 to LOCAL_APPS
- coffeestats.urls:
  - add namespace 'apiv1' below URL path /api/v1
implement explore page

- caffeine.models:
  - add methods recently_joined and longest_joined to CaffeineUserManager
  - add methods latest_caffeine_activity, top_consumers_total and
    top_consumers_average to CaffeineManager
- caffeine.views:
  - implement ExploreView.get_context_data
- coffeestats.settings.base:
  - add django.contrib.humanize to DJANGO_APPS
- add explore page template templates/explore.html
- add snippet for public profile link as templates/includes/profilelink.html