RAA - observable/0.4

A variation on the observer pattern.

Allows the state of a variable to be exposed to observers. An observer registers code to execute when the variable changes (and also a pattern to detect changes of interest). Notification happens when the writer method is called.

New in 0.4:

• Marshal and YAML compatibility: Objects with registered observers can be dumped, but the dumped versions lose their observers, of course.
• API change: Renamed stop_observing to remove_observer.
• API change: signals are now fired by methods of the form #name rather than #name=. There is no longer a reader method associated with the signal (the method that used to be called #name). Signals are intentionally valueless except at the instant of firing, and at that time the value is available as the argument to when_ clauses.
• Added tests.