Split logic from Model, split address into billing, shipping, use functions instead prefixed fields

[devkral]

Instead of using prefixed address fields in Model, use functions to retrieve the data. This way different data sources can be used and a fallback can be provided (BasePaymentWithAddress) in case only one address is available. BasePayment is split in Logic and Model. This allows better tests with less rendundancy. The resulting testmock is in testcommon (test_common would be executed)

[codecov-io]

Codecov Report

Merging #149 into master will increase coverage by 1.68%. The diff coverage is 89.2%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   79.13%   80.81%   +1.68%     
==========================================
  Files          27       28       +1     
  Lines        1649     1757     +108     
  Branches      190      194       +4     
==========================================
+ Hits         1305     1420     +115     
+ Misses        245      237       -8     
- Partials       99      100       +1

Impacted Files	Coverage Δ
payments/sofort/__init__.py	87.71% <ø> (-1.17%)	:arrow_down:
payments/testcommon.py	100% <100%> (ø)
payments/braintree/forms.py	93.1% <100%> (+0.51%)	:arrow_up:
payments/stripe/forms.py	96.92% <100%> (+0.09%)	:arrow_up:
payments/cybersource/__init__.py	66.56% <100%> (+0.1%)	:arrow_up:
payments/authorizenet/__init__.py	79.54% <100%> (ø)	:arrow_up:
payments/sagepay/__init__.py	90.76% <66.66%> (+2.05%)	:arrow_up:
payments/models.py	84.12% <72.91%> (+9.33%)	:arrow_up:
payments/utils.py	91.66% <90%> (-8.34%)	:arrow_down:
... and 1 more

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