[Mercadopago] Add support for mercadopago gateway.

[mandarvaze]

Why?

• Introduced new gateway Mercadopago.
• Added Unit tests.
• Added Integration tests.

[mandarvaze]

Once this PR is merged, I'll close other mercadopago related PRs, mentioning this PR.

[SagarKarwande]

@mandarvaze Can you sync the branch with latest devbranch. Travis build is failing. Commit that has travis changes on the dev branch are not included in the branch.

[mandarvaze]

@mandarvaze Can sync the branch with latest devbranch. Travis build is failing. Commit that has travis changes on the dev branch are not included in the branch.

Done.

[codecov[bot]]

Codecov Report

Merging #185 into dev will increase coverage by 3.30%. The diff coverage is 95.45%.

@@            Coverage Diff             @@
##              dev     #185      +/-   ##
==========================================
+ Coverage   77.39%   80.70%   +3.30%     
==========================================
  Files          14       16       +2     
  Lines         376      456      +80     
==========================================
+ Hits          291      368      +77     
- Misses         85       88       +3     

Impacted Files	Coverage Δ
lib/gringotts/gateways/mercadopago.ex	95.45% <95.45%> (ø)
lib/gringotts/gateways/openpay.ex	97.22% <0.00%> (ø)

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[SagarKarwande]

@mandarvaze Also check if the docs are generating properly.

[ashish173]

@mandarvaze update the README file with the addition of this new gateway.

[mandarvaze]

@mandarvaze update the README file with the addition of this new gateway.

Done.