Support for blocks instead of delegate calls for requests.

[alastairstuart]

It would much easier to keep track of requests if we could add success/completion blocks to them instead of using the delegate call backs. I haven't had time to delve into DAVKit and add them properly so have hacked on a category which can be found here:

https://gist.github.com/4155257

If I have time I'll try and add them in properly.