Adding unit test coverage reporting using coveralls.io

The coverage report on coveralls will look somewhat like this https://coveralls.io/r/phantomas1234/cobrapy

You probably have to change the repo token in .coveralls.yml (sign in on coveralls.io using the opencobra account and add the cobrapy repository to the list of watched repositories to get this token).

You might want to add a couple more omitted directories to the config I guess. I wasn't sure which modules you're aiming for to get a high coverage.

Please, ignore the commit messages. I initially based this branch on my fork's master and had to cherry pick the changes which turned out be a complete nightmare. I might have messed up .travis.yml.