Add a #destroy() method to Annotator

A number of issues have been filed against Annotator (#168, #202, #213)
requesting the ability to recreate an Annotator instance or reload some
component of Annotator. This has been made difficult by the lack of a clean way
of removing existing Annotator instances from the page.

Add a #destroy() method to Annotator which uses the new #removeEvents() method
of Delegator to do the heavy lifting. Plugins have a default #destroy() method
which they can override to do plugin-specific cleanup: see the Filter plugin for
an example.
core: simple instance tracking

It can occasionally be very useful to get a list of instances of Annotator in
the current page. This is currently almost impossible unless you track instances
yourself, which isn't made particularly easy by the default method of
instantiating Annotator, i.e.

    $('p').annotator()

Now we have an explicit #destroy() method, I don't see any particularly harmful
side effects of allowing the Annotator object to keep a reference to each
constructed instance.