Added ability to turn on debug logging via environment variable

For the grunt-karma-coveralls project, I need to be able to turn on
debug level logging but without using `process.argv`. This will allow me
to do so.

This addresses this issue:
https://github.com/mattjmorrison/grunt-karma-coveralls/issues/2

and can be fixed with this commit:
https://github.com/mattjmorrison/grunt-karma-coveralls/commit/4bd6e2b58647dda4a6335fa6077334d8021e23c0