Revisit emiting events.

[lmarkus]

I know a similar issue was closed a year ago. I was hoping it's time to revisit the possibility of Grunt emitting events.

For my particular use case (programmatically load and run gruntfiles) , I would like to see the following two to start with:

• grunt:end: Grunt has drained the queue, and there are no more tasks to run.
• grunt:error: Something blew up.

[vladikoff]

Hey thanks for creating the issue. Marked this for 0.5.0+.