Format project and migrate to CodeCov

[oyeb]

• Updated travis config, added .formater.exs
• Migrate to CodeCov
• Travis will run the formatter check
• Add git-hooks and update contribution guide
• Changed credo max-line-length config from 80 to 100
• Fix various credo warnings

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (dev@a91deec). Click here to learn what that means. The diff coverage is 35.71%.

@@          Coverage Diff           @@
##             dev     #135   +/-   ##
======================================
  Coverage       ?   64.76%           
======================================
  Files          ?       14           
  Lines          ?      474           
  Branches       ?        0           
======================================
  Hits           ?      307           
  Misses         ?      167           
  Partials       ?        0

Impacted Files	Coverage Δ
lib/gringotts.ex	100% <ø> (ø)
lib/gringotts/gateways/cams.ex	93.75% <ø> (ø)
lib/gringotts/response.ex	100% <ø> (ø)
lib/gringotts/gateways/wire_card.ex	0% <0%> (ø)
lib/gringotts/gateways/paymill.ex	0% <0%> (ø)
lib/gringotts/credit_card.ex	100% <100%> (ø)
lib/gringotts/gateways/global_collect.ex	100% <100%> (ø)
lib/gringotts/gateways/authorize_net.ex	97.56% <100%> (ø)
lib/gringotts/gateways/bogus.ex	85.71% <85.71%> (ø)

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