communication between tasks

This problem became apparent in a discussion on the [grunt-contrib-watch](https://github.com/gruntjs/grunt-contrib-watch/issues/14) repository. The issues there was that the watch task can not tell other tasks which files where changed and triggered the watch task to run them.

This shows a general problem in grunt and that is the missing communication infrastructure between tasks. [`grunt.event`](https://github.com/gruntjs/grunt/wiki/grunt.event) provides a good technical implementation and I personally think events are the way to go. But what we need is a solid convention so everybody knows how grunt plugins can be talked to.

I hope we can start a discussion about this matter here.