implement mandatory timezone setting

- caffeine.forms:
  - implement SelectTimeZoneForm
- caffeine.middleware:
  - implement EnforceTimezoneMiddleware to redirect users with no timezone set
    to the timezone selection page after login
- caffeine.urls:
  - add URL pattern 'selecttimezone' for SelectTimeZoneView
- caffeine.views:
  - implement SelectTimeZoneView
- coffeestats.settings.base:
  - add EnforceTimezoneMiddleware to MIDDLEWARE_CLASSES
- add templates/selecttimezone.html with HTML/JavaScript code for timezone
  selection
- requirements/base.txt add pytz to requirements