Add emitError option, update broken nodeUnit tests, update README

The main point of this pull request to to add an option for when a scss-lint fails that allows a developer to do something with the error results. The option is: `emitError` and it utilizes [Grunt's event API](http://gruntjs.com/api/grunt.event#grunt.event.emit) using `grunt.event.emit('scss-lint-error', results);`

I have updated the `README.md` with the new option as well.

Fixes also in this PR include fixing several of the unit tests that were trying to split an empty array, this was returning and error and failing the tests. I have updated with them `!result` for a `pass.scss`. I do not quite follow what `pluralizeMultipleFiles` & `pluralizeSingleFile` are testing, so I did not touch them, but they are still failing the tests.
Sorry I missed this, change 'sass' to 'scss' please.