Added nested endpoints as well as some filters

/api/v1/shows/
/api/v1/shows/?active=true
/api/v1/shows/1/
/api/v1/shows/1/schedules
/api/v1/shows/1/schedules/1
/api/v1/shows/1/schedules/1/timeslots
/api/v1/shows/1/schedules/1/timeslots/1/
/api/v1/shows/1/schedules/1/timeslots/1/notes
/api/v1/shows/1/schedules/1/timeslots/1/notes/1
/api/v1/shows/1/notes
/api/v1/shows/1/notes/1
/api/v1/shows/1/timeslots
/api/v1/shows/1/timeslots/1
/api/v1/shows/1/timeslots/1/notes
/api/v1/shows/1/timeslots/1/notes/1

On every /timeslot endpoint, filter variables ?start=2017-01-01&end=2017-12-31 are available.

Properly create, update and delete:

* Users
  - only superusers may create
  - common users can only retrieve/update themselves
  - update is constrained to certain non-crucial fields for non-superusers

* Shows
  - only superusers may create
  - update is constrained to certain fields for non-superusers

* Notes
  - creating/updating is constrained to shows and timeslots the non-superuser owns

* Schedules
  - only superusers may add/update/delete
  - collision detection still needed

* Timeslots
  - direct creation is prohibited for everyone (only by adding/updating schedules)
  - update is constrained to certain fields
  - only superusers may delete


Available GET endpoints for (only superusers may add/update/delete)

* Hosts
* Types
* Topics
* Categories
* RTRCategories
* Languages


See #22 #23