Add #until method to Property

This allows a Property to switch 'behavior' when the
a stream has an event. For example `a.until(stream, b).now` would
return the value of `a` until the stream occurred and it would
then return the value of `b`.
Add #toggle for Property

Similar to #until except for that each occurence will toggle
between the Properties.
Make UntilProperty extend Property

We're not in a static language anymore... :unamused:
Merge pull request #22 from seadowg/until

Add #until and #toggle to Property