added an adaptor for angular scope events to EventStreams

[ajkovar]

Maybe I was overlooking something really obvious but I was having a hard time figuring out how to transform angular scope events into EventStreams.

I think it would be nice to be able to do things like this:

stream.takeUntil($scope.eventAsStream("$destroy").map(true))

Let me know if there is some other obvious way to handle this, otherwise please consider this pull request!

[ajkovar]

Good suggestion! I added it.

[ajkovar]

My previous deregistration logic was flawed. I added a fix for it.