Add Paydirekt provider

[devkral]

This adds a backend for paydirekt. It depends on some other pullrequests of mine: https://github.com/mirumee/django-payments/pull/148 https://github.com/mirumee/django-payments/pull/149

Sry for the long commit history. I required the backend for a project.

[codecov-io]

Codecov Report

Merging #155 into master will increase coverage by 0.08%. The diff coverage is 79.11%.

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
+ Coverage   79.13%   79.22%   +0.08%     
==========================================
  Files          27       29       +2     
  Lines        1649     1993     +344     
  Branches      190      234      +44     
==========================================
+ Hits         1305     1579     +274     
- Misses        245      292      +47     
- Partials       99      122      +23

Impacted Files	Coverage Δ
payments/sofort/__init__.py	87.71% <ø> (-1.17%)	:arrow_down:
payments/paypal/__init__.py	84.58% <100%> (ø)	:arrow_up:
payments/core.py	83.07% <100%> (-4.62%)	:arrow_down:
payments/authorizenet/__init__.py	79.54% <100%> (ø)	:arrow_up:
payments/stripe/forms.py	96.92% <100%> (+0.09%)	:arrow_up:
payments/braintree/forms.py	93.1% <100%> (+0.51%)	:arrow_up:
payments/stripe/__init__.py	60% <100%> (ø)	:arrow_up:
payments/dummy/__init__.py	81.13% <100%> (-5.67%)	:arrow_down:
payments/cybersource/__init__.py	66.56% <100%> (+0.1%)	:arrow_up:
payments/sagepay/__init__.py	90.76% <66.66%> (+2.05%)	:arrow_up:
... and 9 more

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[patrys]

Thank you for this contribution. I think we'd prefer for new payment providers to live in their own packages. In fact we've discussed splitting the current main package.

Please take a look at https://github.com/esistgut/django-payments-bnlepos for an externally-maintained payment provider.

[devkral]

good idea