Async: Get tests working again

[helje5]

Would be cool to have tests ;-)

Related to swift-server#96:

It would be awesome if you would get the tests working again. I don't plan to work on such any time soon, so feel free to do anything you want!

Adjusting the tests might be challenging, because, well, they need to be asynchronous. For Noze.io I have a helper class which tries to deal with the asynchronicity, not sure whether it might help you or not (or even whether this is the correct way to test that stuff): NozeIOTestCase.swift

The other hurdle is that I made the HTTPResponseWriter concrete (i.e. it is not a protocol anymore). I suppose that makes some mocking hard. We could change it back to a protocol, but I think it was suggested that we should avoid them for performance reasons.

[carlbrown]

Added #13 for Mac tests