Add slather for test coverage

If you merge in this PR and enable your repo at http://www.coveralls.io, you can have a badge like this on your README :wink: 

[![Coverage Status](https://coveralls.io/repos/marklarr/ocmock/badge.png?branch=marklarr%2Ftest_coverage)](https://coveralls.io/r/marklarr/ocmock?branch=marklarr%2Ftest_coverage)