[global-collect] Layout, docs improvements and code refactors

[jyotigautam]

• credo issues resolved
• Fixed some layout issues
• Corrected amount in examples
• Ran the elixir 1.6 code formatter
  • Used sigils in mocks
• Removed unnecessary functions
  • Reduced arity of add_money
• Refactored Timex usage
• Removed a test on validate_config as it is already tested.

[oyeb]

• It appears that GlobalCollect requires that some optional params be set, but this has not been documented.
• Some optional params are assumed to be present and their absence is not handled ie, a null is rendered to the request.
• The fact that GC works in cents is not documented.

[codecov-io]

Codecov Report

Merging #111 into dev will decrease coverage by 0.6%. The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #111      +/-   ##
==========================================
- Coverage   64.76%   64.16%   -0.61%     
==========================================
  Files          14       14              
  Lines         474      466       -8     
==========================================
- Hits          307      299       -8     
  Misses        167      167

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

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