fixed gc for Timespans and StreamStatus

Garbage collection requires weak refs
added registration for subconverters
try new base.getContextByClass
update test results

one error is due to the temporary commit of a function not 100% working…
fixed cached time in seconds
move SlottedObject to common
move Site/Sites to sites.py

Also changed Sites._definedContexts to Sites.siteDict. Will make public
other attributes later.
updates to timespan/tests in stream.__init__
new getContextByClass using Timespans
new GetContextByClass! FASTFASTFAST
terminology: Site is the Stream Object.

The Container is now called SiteRef
continue term. clarification; weakref speedup
1.9.3 - Pitch is not a Music21Object

25% speedup.
fix some doc errors in usersGuide
test suite running in 3/5ths the time as before.
del classCache

a great idea, but timespans implement the idea better.
clean up freezeThaw a bit

but not as much as I would like. The removeStreamStatusClient needs to
be there for a bit until we can really have a stream.__getstate__ etc.
rem unwrapWeakref freezeIds from Var/Span

not used anymore, unlikely to be used in the future.
all tests pass
clean up jsonpickle warnings in eclipse