- restore mapper.get_property() to use the _props dict. at the moment

- restore mapper.get_property() to use the _props dict.   at the moment
synonyms for relationships might just be taken out altogether, since they aren't
documented and are of little use.   a plain proxying descriptor, combined with
attribute-based usage with Query (as opposted to naming it by string)
 can do the same thing more simply.
- add event support to composites, change the model around so that the composite
is generated at the point of load.
- add a recipe for tracking mutations on composites.  will probably make both
of these mutations examples into extensions since they're intricate, should
have a lot of test coverage, and what they need to do is fairly straightforward.
Will use metaclasses so that no extra userland step is needed beyond usage
of the type.