add test to reproduce issue #13
support empty date and time submissions for #13

- caffeine.forms:
  - import timezone
  - rename timezone variable to cleantimezone
  - make date and time in SubmitCaffeineForm optional
  - react on empty date or time by using server time converted to user's
    timezone
merge bugfix branch, fixes #13
Merge branch 'release/0.1.6' into live