Historical coverage stats

The reason for this pull request can be seen [here](https://coveralls.io/r/s0undt3ch/salt).

An idea:
![coverage](https://f.cloud.github.com/assets/300048/588037/206699d8-c977-11e2-9407-ef33301540a9.png)

It's already enabled for Salt Stack's salt repository.
At least it will allows up to get a notion of our test suite coverage.

Of course, the coverage **will never** be accurate since we would need to merge the coverage data from multiple distributions :smile: