eliminated the relationship between Plugin's and PluginFamily.  Cleans up PluginFamily substantially
put an IObjectBuilder interface into IBuildSession
removed PluginFamily.DefaultKey, eliminated [PluginFamily(DefaultKey=blah)]
thinning down PluginFamily.Seal() so that PluginFamily makes the default lazy
eliminated the old SystemRegistry and internal system container
removed IGraphBuilder.FinishFamilies
eliminated the old PluginGraphBuilder.BuildFromXml stuff
streamlined the bootstrapping process
made the PluginFamily do the default instance determination a little smarter
more cleanup on PluginFamily