Add test coverage

[ocramz]

Code coverage should be added to the Travis config (perhaps the cabal file and/or the stack options need to be changed in order to account for hpc coverage generation); currently in Travis there is only a project key. This tool uploads hpc coverage reports to codecov.io .

[mjarosie]

Hi, I'd like to try my best to contribute. I'll fill in a pull request when ready : )