Code coverage with SimpleCov and Coveralls

This PR will add code coverage reports.  It will produce a SimpleCov report when you run locally (at coverage/index.html).  It should also hook Travis runs into [Coveralls](https://coveralls.io), which in turn will add comments to Pull Requests to indicate if coverage is increasing, decreasing or remaining the same.  (See [this example](https://github.com/lemurheavy/coveralls-ruby/pull/18)).

In case anyone is wondering, I'm not proposing there should be a code coverage target.  I agree with the Martin Fowler [post on test coverage](http://martinfowler.com/bliki/TestCoverage.html) which explains:

>Test coverage is a useful tool for finding untested parts of a codebase. Test coverage is of little use as a numeric statement of how good your tests are.