feat #13 console report enhancements

Added:
- print remaining tests countdown (per-browser and total)
- print browser name and version
- print test elapsed time
- mark long-running tests (over configurable threshold) with a different color
- logging of connected and better logging of disconnected slaves

Remaining tests countdown can be slightly imprecise because we decrement at task
finish but print on test finish, but it's good enough to not complicate
the code unnecessarily.

Close #13. Close #29.