Add code coverage reporting

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Adds code coverage reporting using Coveralls. This uses the existing code coverage tools, it just adds a final step to Travis CI to report the lcov.info information to coveralls on success.

To see what the coveralls tool will look like, you can see my fork.

By default, all pull requests are commented on with their code coverage. Here is an example. This behavior can be disabled.

Note: To enable coveralls integration with this original repo, it does require you to log into coveralls.io and allow github integration. The account is free, but I understand if that isn't something you want.

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I rebased this pull request with the latest from master. If you merge this pull request and log into Coveralls to allow github integration (free)- then all future pull requests will get a comment saying if the change decreases or increases code coverage. This can be useful to encourage future contributors to write unit tests.

Current code coverage:

Statements   : 100% ( 211/211 )
Branches     : 95.52% ( 64/67 )
Functions    : 100% ( 43/43 )
Lines        : 100% ( 211/211 )