added an adaptor for angular scope events to EventStreams

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!