Feature: capture exceptions

The problem: In your jasmine specs, you introduce an error in your async method callback handler, 
and all you get is "Fatal error: Object #<Object> has no method 'nullMethod' (for example) without an error stack trace.

The reason why you don't get the stack trace from jasmine, is because 1) the method is async and 2) Grunt is handling this uncaught exception, and then exiting the process without printing the stack trace. 

One solution would be to run grunt with the --stack option, but it is not obvious. Another option, is to handle this uncaught exception from within the grunt-jasmine-node task.

I believe this issue has caused some people a lot of pain, and the introduction of this option will make life easier for new users. Alternatively, you should update the README to explain *why* you won't get a stack trace when you introduce an error in an async handler, and want you can do to display the stack trace.