resolves #211: run tests in parallel

@kevgo @allewun 

Type | Run 1 | Run 2 | Run 3 
--- | --- | --- | --- |
Without parallel | 2:24 | 2:26 | 2:22 
With parallel (8 processors) | 0:36 | 0:38 | 0:37 

* Had to give each thread its own repository base so they don't overwrite each other.
* Moved formatter to `rerun` (prints just the failing tests) as the other formatters don't look very good when outputting in parallel.
* Removed fuubar formatter as not used anymore (does not work well with parallel)

I suggest using the "progress" formatter. It at least shows some progress. I strongly dislike commands just not showing something for a minute, and I don't know if its hanging or still running.