[PinPayments] purchase and refund function with test cases

[ravirocx]

PinPayments

purchase/3

• Implemented two purchase function:
  1. With card params
  2. With card Token

refund/2

• Implemented refund function

Docs

• Added docs for purchase/3.
• Added docs for refund/3

Tests

• Added integration tests for purchase/3, using ExVCR.
• Added integration tests for refund/2, using ExVCR.

function store/2 is also added, as the purchase was dependent on store

[codecov-io]

Codecov Report

Merging #146 into pinpay will increase coverage by 6.11%. The diff coverage is 80.43%.

@@            Coverage Diff             @@
##           pinpay     #146      +/-   ##
==========================================
+ Coverage   60.04%   66.15%   +6.11%     
==========================================
  Files          15       15              
  Lines         498      520      +22     
==========================================
+ Hits          299      344      +45     
+ Misses        199      176      -23

Impacted Files	Coverage Δ
lib/gringotts/gateways/pin_payments.ex	80.43% <80.43%> (+80.43%)	:arrow_up:
lib/gringotts/gateways/global_collect.ex	100% <0%> (ø)	:arrow_up:

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