Optimize console output

This bundles several commits. The first makes functional changes. The 2nd and 3rd remove some unnecessary code. The fourth adds jshint and fixes a bunch of issues. These should be reviewed separately.

I've ran `npm test` to check the few mocha unit tests and run jshint. jshint still fails with one error which I didn't know how to resolve (see utils.js line 10/11).

I've also ran `ruby tests/test.rb`, runs fine.

And I've ran a testsuite on another project. Here's a screenshot where one of the two workers has a failure (on purpose), showing the new console output:
![...](http://bassistance.de/i/c018b6.png)

Details from the 1st commit:

* Only log screenshot URLs for failed tests, still always create them, can always be accessed on browserstack.com/automate.
* Degrade the "Exiting" logs to debug, not needed in normal operations.
* Update logs for finished workers to use less color, slightly more compact format.
* Fix bug in formatTraceback, where a falsy actual or expected value would cause nothing to be printed.
Can we have this as under single quotes too?