ORM integration

[sergei-maertens]

Adds support for mapping value -> order in querysets

[codecov-io]

Codecov Report

Merging #63 into develop will increase coverage by 0.18%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop      #63      +/-   ##
===========================================
+ Coverage    96.36%   96.55%   +0.18%     
===========================================
  Files            1        1              
  Lines          110      116       +6     
  Branches        16       17       +1     
===========================================
+ Hits           106      112       +6     
  Misses           3        3              
  Partials         1        1

Impacted Files	Coverage Δ
djchoices/choices.py	96.55% <100%> (+0.18%)	⬆️

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[coveralls]

Coverage increased (+0.1%) to 97.521% when pulling 9a6f68a on sergei-maertens:feature/orm-integration into 7cb6c0e on bigjason:develop.

[coveralls]

Coverage increased (+0.1%) to 97.521% when pulling 117a08a on sergei-maertens:feature/orm-integration into 7cb6c0e on bigjason:develop.