Functional updates & bug fixes

[PiDelport]

This adds a number of functional updates and bug fixes:

• New setting: PAYFAST_BASE_URL
• Drop support for Django < 1.7 and South
• m_payment_id is now supports the full range of alphanumeric strings that PayFast allows, and can be passed to PayFastForm to constrain PayFastOrder creation.
• ITN signature checking and validation calls (USE_POSTBACK) should now work again.

[codecov[bot]]

Codecov Report

Merging #3 into master will decrease coverage by 0.64%. The diff coverage is 65.27%.

@@           Coverage Diff           @@
##           master    #3      +/-   ##
=======================================
- Coverage   87.64%   87%   -0.65%     
=======================================
  Files          12    12              
  Lines         348   377      +29     
  Branches       21    28       +7     
=======================================
+ Hits          305   328      +23     
- Misses         36    38       +2     
- Partials        7    11       +4

Impacted Files	Coverage Δ
payfast/api.py	56.66% <0%> (-1.96%)	:arrow_down:
payfast/views.py	100% <100%> (ø)	:arrow_up:
payfast/tests.py	100% <100%> (ø)	:arrow_up:
...igrations/0003_update_payfastorder_m_payment_id.py	56.25% <56.25%> (ø)
payfast/forms.py	81.35% <57.14%> (-5.44%)	:arrow_down:
payfast/models.py	98.18% <66.66%> (-0.04%)	:arrow_down:
payfast/conf.py	76.19% <66.66%> (-6.17%)	:arrow_down:

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