Added Codecov report

[BartBucknill]

So that we can have a codecov report with PRs and for a button in readme; changes:

• added codecov to travis.yml
• add istanbul coverage report to test script Reference dwyl tutorial. All tests pass.

  45

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@8a2deec). Click here to learn what that means. The diff coverage is n/a.

@@          Coverage Diff           @@
##             master   #46   +/-   ##
======================================
  Coverage          ?   96%           
======================================
  Files             ?     5           
  Lines             ?    75           
  Branches          ?     0           
======================================
  Hits              ?    72           
  Misses            ?     3           
  Partials          ?     0

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Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 8a2deec...82ddd11. Read the comment docs.

[rebecacalvoquintero]

yayy