Mercadopago support

[WhyNotHugo]

Add support for MercadoPago (one of the largest payment processor in south america).

I've been using this live on a few websites for almost a year, and it's been working perfectly. I only haven't merged it due to missing tests, since I'd like to have proper tests before merging.

I did a lot of refactors, polishing and improvement, so had to re-write tests. Will attempt to write integration tests in future too.

[codecov]

Codecov Report

Merging #232 (277bb88) into master (8fe674c) will increase coverage by 0.41%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
+ Coverage   79.50%   79.92%   +0.41%     
==========================================
  Files          27       28       +1     
  Lines        1688     1783      +95     
  Branches      196      212      +16     
==========================================
+ Hits         1342     1425      +83     
- Misses        244      251       +7     
- Partials      102      107       +5     

Impacted Files	Coverage Δ
payments/mercadopago/__init__.py	87.23% <87.23%> (ø)
payments/models.py	76.00% <100.00%> (+0.19%)	⬆️

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