Using composite dispose in takeUntil.

This changes the semantics of unsubscribing slightly, though it appears to me to be in a desirable way, but if there is a reason for not unsubscribing to both the src and stopper at each point, please undo this commit.
Use composite disposable in flatmap
Rename to compositeUnsubscribe and export
Rename from to update
Undoing accidental change in scheduler
Handle properties in join patterns. These will not be part of the
synchronization logic, but will simply be available as sampled values.

BREAKING CHANGE: Zip is implemented in terms of Bacon.when, and will
now react differently with properties as arguments. I think this is a
preferrable behaviour however, and since zip is recently added I
suggest we change the semantics.
Adding tests