Test coverage

Add test coverage using [cloverage](https://github.com/lshift/cloverage) and [coveralls.io](https://coveralls.io).

I had to refactor existing tests so they now work with either `lein midje` or `lein test`. This made them slightly less pretty, but provides excellent support for many libraries assuming the use of the standard test library.

To improve our poor coverage slightly, I took the liberty of testing `tip.core/-main` as well.