this adds the fields `created_at`, `created_by`, `updated_at` and `updated_by`

Close #51

See #21 #23

Shall we still put this in a sub app?
Would still need to figure out how to hook from the sub app into the serializer viewsets (and not vice versa) in order the custom fields get saved.

See #21

Updating is constrained to defined fields.
See #22

Implemented django-oauth-toolkit for authentication.

Working on prototype for playout API (momentarily still called week_schedule())

Added permission restrictions for common users in APIs
See #23

Added playlist_id to TimeSlot datamodel
See #21

Made some minor changes in calendar.

See #23

- Let them only edit their own user profile and only certain fields
- Let them only edit shows they own and only certain fields
- Let them only edit own notes and only assign them to shows and timeslots they own

Removed some unused code and added some help texts to form fields.

* Extended datamodel of user profile by introducing "profile"-subapp
* Implemented thumbnail generation for shows, notes and user profile using django-versatileimagefield (see requirements.txt)

See #21