Stop failing on warnings by default

I was a bit surprised when this plugin failed on warnings:

- usually things that issue warnings and errors, like compilers or [grunt-coffeelint](https://github.com/vojtajina/grunt-coffeelint), only fail on errors by default
- it seems to defeat the purpose of having 2 levels of reporting if we act the same for both

As of this PR, if you accept it:

- `coffeelint.success` is true unless there are errors, even if there are warnings
- hence, without any change to the `fail` reporter, it doesn't fail on warnings anymore
- there is a new `failOnWarning` reporter to reproduce the old behaviour

Also, this is based off janraasch/gulp-coffeelint#18 to include the output (IMESHO) improvement but let me know if you're keen on rejecting #18 and accepting this one, I'll be happy to change it.

Edit: It also fixes a small problem I spotted in the reporter spec, with a stub not being reset across tests, see f26bad9