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.
Log unfinished tests instead of undispatched

We used to log the number of tests that have not started yet instead of
ones that were finished, which led to incorrect numbers especially when
using multiple browsers at the same time.

Now this is corrected both in the console and Attester status page.
Log campaign elapsed time at the end
Browser detection by operating system

Close #38.