Added deleted? method to Gliffy::Document
Document now is Observable.

Currently,  :delete  notification sent  to  observers  when document  is
deleted.
Updated Document Observable contract

Added reference to the target document to :delete notification.
Updated specs.

Specs now  correctly reflect the fact  :delete event has a  reference to
the target document.
Folder now listens to Document events.

When :delete event is received, folder removes document for the document
list transparently.