Add PayU provider

[PetrDlouhy]

No description provided.

[codecov]

Codecov Report

Merging #239 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #239   +/-   ##
=======================================
  Coverage   79.64%   79.64%           
=======================================
  Files          27       27           
  Lines        1621     1621           
  Branches      191      191           
=======================================
  Hits         1291     1291           
  Misses        227      227           
  Partials      103      103           

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[WhyNotHugo]

Thanks!

[PetrDlouhy]

@WhyNotHugo @patrys Could you please take look also on #217, which is needed by the PayU backend to be used without need to use non-django-payments Payment model?