Feature/add code coverage

[benvenutti]

Hey there, here is a small contribution adding code coverage for ni-media. Here is an example of the generated report: https://codecov.io/gh/benvenutti/ni-media/branch/feature%2Fadd-code-coverage

Changes:

• add a Coverage build flavor
• added a build for coverage (gcc-5 and gcov-5) on Travis
• codecov.yml added to filter out source files that should not be in the report
• reports are uploaded to codecov.io
• coverage badge on README

You will notice that the PCM tests were disabled for the coverage build. For proper coverage, optimization has to be turned off. This was causing the PCM tests to run for a really long time and Travis has a limit of 10 minutes to run without any outputs. So this causing the actual coverage build to fail. Here is an example of the problem on Travis: https://travis-ci.org/benvenutti/ni-media/jobs/522984435

@marcrambo, you will have to create an account on codecov.io and enable ni-media there. Then, once the PR triggers the build, you will get the first report in your codecov.io. There you can get the correct badge URL to your repo. We then need to update the URLs in the README file in this PR, so the repo points to its respective reports.

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@7365043). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #28   +/-   ##
=========================================
  Coverage          ?   86.93%           
=========================================
  Files             ?       94           
  Lines             ?     1745           
  Branches          ?        0           
=========================================
  Hits              ?     1517           
  Misses            ?      228           
  Partials          ?        0

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