[Mercadopago] Implemented purchase

[subpal]

1. Implemented purchase
2. Added test cases for purchase
3. Added docs for purchase

[codecov-io]

Codecov Report

Merging #154 into mercadopago will increase coverage by 8.83%. The diff coverage is 77.2%.

@@               Coverage Diff               @@
##           mercadopago     #154      +/-   ##
===============================================
+ Coverage        57.62%   66.46%   +8.83%     
===============================================
  Files               15       15              
  Lines              472      501      +29     
===============================================
+ Hits               272      333      +61     
+ Misses             200      168      -32

Impacted Files	Coverage Δ
lib/gringotts/response.ex	100% <ø> (ø)	:arrow_up:
lib/mix/new.ex	0% <0%> (ø)	:arrow_up:
lib/gringotts/gateways/paymill.ex	0% <0%> (-100%)	:arrow_down:
lib/gringotts/gateways/wire_card.ex	0% <0%> (ø)	:arrow_up:
lib/gringotts/credit_card.ex	100% <100%> (ø)	:arrow_up:
lib/gringotts.ex	100% <100%> (+100%)	:arrow_up:
lib/gringotts/gateways/global_collect.ex	100% <100%> (ø)	:arrow_up:
lib/gringotts/gateways/stripe.ex	71.79% <100%> (+71.79%)	:arrow_up:
lib/gringotts/gateways/trexle.ex	100% <100%> (ø)	:arrow_up:
lib/gringotts/money.ex	70% <70%> (ø)	:arrow_up:
... and 10 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update b8692ba...7b2b36a. Read the comment docs.

[mandarvaze]

Superceded by #185