add some asynchronous callbacks for aioshutil

[Vizonex]

I found this library after seeing a draft from multidict so I thought about contributing over here since I got rather curious.

I had an idea for making aioshutil support asynchronous callbacks but I will still say my idea/concept is in it's beta-phase.
It may require getting a bit deeper into shutil's actual code depending on some different circumstances.

So far I have done rmtree and added in an asynchronous callback but so far I think I would like for that callback to be awaited upon so it may need further implementing such as having internal calls to some different os functions but via using eventloop executors.
I wanted to do the same with copyfileobj since that seems to be an issue at the moment perhaps there could be a secondary function just for that functionality maybe?

fixes #12