Added a wait option to be used in combination with background:true

This allows you to run several karma tasks in the same queue without getting multiple the port in use warning. 

https://travis-ci.org/mokkabonna/knockout.bindingHandlers.number#L2335

Also causes possible eventemitter warnings

https://travis-ci.org/mokkabonna/knockout.bindingHandlers.number#L2174


This would preferrably be implemented without needing it to run in the background. If karma [supported being stopping](https://github.com/karma-runner/karma/issues/136) without the whole process exiting then you could implement it like so:

	server.start(data, function(code){
		server.stop();
		done(code === 0);
	});

You would also get the benefit of getting the output from karma as it comes, not all at once like now with this wait option.

But until then I hope this is something you could want to merge. 

If so let me know and I'll add the docs for the wait option as well.