Cover all entities in lookup functions

[Borewit]

If possible, add lookup functions for the following entities can be looked up: Lookup functions

• /ws/2/area
  • [x] Implementation
  • [x] Unit tests
  • [x] Documentation
• /ws/2/artist
  • [x] Implementation
  • [x] Unit tests
  • [x] Documentation
• /ws/2/collection
  • [ ] Implementation
  • [ ] Unit tests
  • [ ] Documentation
• /ws/2/event
  • [x] Implementation
  • [x] Unit tests
  • [ ] Documentation
• /ws/2/instrument
  • [ ] Implementation
  • [ ] Unit tests
  • [ ] Documentation
• /ws/2/label
  • [ ] Implementation
  • [ ] Unit tests
  • [ ] Documentation
• /ws/2/place
  • [ ] Implementation
  • [ ] Unit tests
  • [ ] Documentation
• /ws/2/recording
  • [x] Implementation
  • [x] Unit tests
  • [x] Documentation
• /ws/2/release
  • [x] Implementation
  • [x] Unit tests
  • [x] Documentation
• /ws/2/release-group
  • [x] Implementation
  • [x] Unit tests
  • [x] Documentation
• /ws/2/series
  • [ ] Implementation
  • [ ] Unit tests
  • [ ] Documentation
• /ws/2/work
  • [x] Implementation
  • [x] Unit tests
  • [x] Documentation
• /ws/2/url
  • [ ] Implementation
  • [ ] Unit tests
  • [ ] Documentation

Example API call: http://musicbrainz.org/ws/2/area/8a754a16-0027-3a29-b6d7-2b40ea0481ed

Ref: https://wiki.musicbrainz.org/Development/XML_Web_Service/Version_2#Subqueries