- Remove unused imports,
- Use HyperlinkedModelSerializer,
- Hide the internal ID field,
- Hide fingerprint,
- Cover more endpoints with tests,
- Remove as much untested code as possible,
- Clarify which URL is for what resource (ie. ProposalDetail didn't list
  votes but ProposalList did, ending up in horrible performance),
- Change tests so that they query indented responses, making diff'ing
  easier

It's used to sort Votes, takes votes list of 10 items in the API from 5s
to 300ms.

Code by @luxcem, left almost untouched.

remove_dossiers_without_recommendation and
remove_representatives_without_votes are useful to clean the database
before dumping test fixtures.

Also:

- code was PEP8'ed
- tests were added,
- untested code was removed,
- setup.py was fixed and requirements.txt is gone,