Add codecov integration to repository

[ryukinix]

• codecov trigger on .travis.yaml
• codecov badge on README.md

Related to #13.

Please, don't merge this before #68.

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@0585bd6). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #69   +/-   ##
=========================================
  Coverage          ?   63.02%           
=========================================
  Files             ?        9           
  Lines             ?      238           
  Branches          ?        0           
=========================================
  Hits              ?      150           
  Misses            ?       88           
  Partials          ?        0

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[ryukinix]

As this is the first codecov PR, this is ok to failure about the base branch don't have that to compare. I'll merge after #68