add integration with django-autocomplete-light

[MRoci]

Since the improvements due to the autocompletion feature are fundamental to have an usable user experience with a very large user base i think it would be nice to have django-autocomplete-light support already built into the library as well as for django-ajax-selects. This could lead in a smoother integration and having fewer dependencies to mantain in projects that heavily rely on dal.

This PR looks for a setting key called SU_DAL_VIEW_NAME and if present it tries to use the dal autocompletion widget in the form.

Maybe the configuration work documented in the README could be reduced if we brought the autocompletion view into the library as well

[coveralls]

Coverage: 91.85% (-2.6%) from 94.426% when pulling 8b7394cc8151b434650f26f6faf706c2eaef5e74 on MRoci:dal-integration into aadabf6aa395060db2d5f626372b981d80f0a0f7 on adamcharnock:develop.

[coveralls]

Coverage remained the same at 0.0% when pulling 5cd146c3e2372a7a2d95004c84616e172d8263cc on MRoci:dal-integration into 856efc83fade37fbec63dc3672ecaca90fd42211 on adamcharnock:develop.

[PetrDlouhy]

I have updated the whole codebase and added testing in all supported versions of Django. Could you please rebase this to the current develop branch and fix the tests, if there are any problems?

[MRoci]

@PetrDlouhy with a little bit of latency i've rebased this PR against current develop :sweat_smile:

[PetrDlouhy]

@MRoci I merged #83, now you can rebase once more to see if the tests are passing.

[codecov-commenter]

Codecov Report

Patch coverage: 43.75% and project coverage change: -2.57 :warning:

Comparison is base (aadabf6) 94.46% compared to head (eade02f) 91.90%.

:exclamation: Current head eade02f differs from pull request most recent head 8b7394c. Consider uploading reports for the commit 8b7394c to get more accurate results

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #74      +/-   ##
===========================================
- Coverage    94.46%   91.90%   -2.57%     
===========================================
  Files            9        9              
  Lines          307      321      +14     
===========================================
+ Hits           290      295       +5     
- Misses          17       26       +9     

Impacted Files	Coverage Δ
django_su/forms.py	55.26% <20.00%> (-12.60%)	:arrow_down:
django_su/views.py	89.65% <83.33%> (-1.09%)	:arrow_down:

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[PetrDlouhy]

@MRoci Thank you very much for your contribution.

Can you please also add some tests for the new code?