Add json api settings module

[sliverc]

This is inspired by api_settings of REST framework.

Advantages:

• Centralizing all json api settings and defaults in one module
• Simplifying refactoring and deprecation of settings in the future
• Properly use override_settings in tests instead of monkey patching

[codecov-io]

Codecov Report

Merging #419 into master will increase coverage by 0.08%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master     #419      +/-   ##
==========================================
+ Coverage   91.75%   91.83%   +0.08%     
==========================================
  Files          55       57       +2     
  Lines        2923     2953      +30     
==========================================
+ Hits         2682     2712      +30     
  Misses        241      241

Impacted Files	Coverage Δ
example/tests/test_model_viewsets.py	100% <100%> (ø)	⬆️
rest_framework_json_api/settings.py	100% <100%> (ø)
example/tests/unit/test_settings.py	100% <100%> (ø)
rest_framework_json_api/parsers.py	97.14% <100%> (ø)	⬆️
rest_framework_json_api/exceptions.py	84.61% <100%> (ø)	⬆️
example/tests/test_generic_viewset.py	100% <100%> (ø)	⬆️
example/tests/unit/test_utils.py	100% <100%> (ø)	⬆️
rest_framework_json_api/utils.py	90.75% <71.42%> (+0.03%)	⬆️

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[sliverc]

@n2ygk Could you have a look at this? Thanks.

[n2ygk]

This looks good as an intermediate step to perhaps a future (breaking) version in which these settings are in a dict instead of having a common prefix (as is done in DRF):

JSON_API_FORMAT_KEYS = 'camelize'
JSON_API_FORMAT_TYPES = 'camelize'
REST_FRAMEWORK = {
    'PAGE_SIZE': 5,
    ...

[sliverc]

Thanks for review. Merging it therefore. Yep, we could certainly think about moving to a dict approach as DRF does. One step at the time... ;).