Initial implementation of AsyncSubject

I've done some work on AsyncSubject<T>, including unit tests and I'm looking for feedback. 

I don't expect to get this pull request immediately accepted. But I hope to find out if this is the correct workflow for this repository, and if the code is up to your standards, before I start pouring in more hours into issue #17. If you guys are ok with the AsyncSubject<T> implementation I'll start working on the others.

Looking forward to some feedback!