New operation Finally (issue #43)

Hi,

I was looking around for some fun stuff to do for the Netflix Cloud Prize, and I figured I might start by contributing some enhancements to RxJava.

I did my best to match your existing style and practices, but I've doubtless made some mistakes.  If you have time to take a quick look and give me some feedback, I'll accept it gratefully. 

Cheers,
--abliss