Git tag feature

[fedefernandez]

Fixes #68

Also adds the createReference op

Please @juanpedromoreno could you review? Thanks!

[codecov-io]

Codecov Report

Merging #71 into master will decrease coverage by 1.3%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
- Coverage   80.04%   78.74%   -1.31%     
==========================================
  Files          34       34              
  Lines         471      795     +324     
  Branches        2        4       +2     
==========================================
+ Hits          377      626     +249     
- Misses         94      169      +75

Impacted Files	Coverage Δ
.../main/scala/github4s/free/domain/PullRequest.scala	100% <ø> (ø)	⬆️
.../src/main/scala/github4s/free/domain/GitData.scala	0% <ø> (ø)	⬆️
.../main/scala/github4s/free/algebra/GitDataOps.scala	33.33% <0%> (-9.53%)	⬇️
...4s/shared/src/main/scala/github4s/GithubAPIs.scala	95.55% <100%> (+0.31%)	⬆️
...s/shared/src/main/scala/github4s/api/GitData.scala	100% <100%> (ø)	⬆️
...cala/github4s/free/interpreters/Interpreters.scala	84.44% <33.33%> (-3.32%)	⬇️
...thub4s/shared/src/main/scala/github4s/Github.scala	81.81% <0%> (-1.52%)	⬇️
...ub4s/shared/src/main/scala/github4s/Decoders.scala	95.09% <0%> (-0.74%)	⬇️
...scala/github4s/HttpRequestBuilderExtensionJS.scala	0% <0%> (ø)	⬆️
... and 30 more

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[juanpedromoreno]

LGTM!

Codecov and Travis are complaining though :P

[juanpedromoreno]

🤔