RAA - observable/0.3

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 0.3:

- Now supports distributed observers with DRb. See the chat-server.rb
and chat-client.rb examples in the FoxTails package (0.2).

- handle_observer_exception: a mechanism for handling exceptions in observer call-backs.

- Having an observable attr called var also gives you stop_observing_var.

- Signal handlers now complain about reentrant calls.

- Added a few tests.

- Added a little more documentation.