Add support for log levels (with suppressable output) to IndentingWriter

To be used in the unit testing harness.
Only log passing unit tests in verbose mode

... which is introduced as an `--verbose` option to the grunt setup.

Plus, quit with an error in case of failing unit tests.

Without this, failing unit tests are swallowed in a sea of useless spew, making them very easy to miss.

Also restores the ability to specify which unit tests should be run by using the `--tests=path/to/some/js/files` syntax.
Merge pull request #1562 from tschneidereit/master

Reduce spew from shell unit tests