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.

Conflicts:
	src/annotator.coffee
close ck--/ann#396 [typed-packaging] remove all events
ck--/ann#396 [typed-packaging] remove TextHighlight@_init events

this is a hack but that should be ok for now