Remove callback async code

One for review - what do you think?

I have deleted all the callback async code from `wrapper.PleaseAsync(x => doSomething(x));` on down because:

- We know that this code is not used by us, probably not used elsewhere, and maintaining it is an overhead.
- If you want to async all the api calls, the async_dotnet45 branch gives you task-basked asynchrony (`async ... await`) and more readable code. Callback async is out of favour in c# now. 

The advantage of the code removed here is that it lets you async *some* calls, and do it in .net 3.5 not 4.5. But AFAIK nobody is doing that.