fixed and improved test_model_pickle
fix pickle/copy issues with some component types

Always set _export=False in the pickle protocol methods __getstate__ and
__getnewargs__, which will keep SelfExport.export from being called on
the objects upon reinstantiation. Also, it actually seems wrong to re-export
such reinstantiated objects unconditionally anyway -- leave it to higher-level
code to make that decision.
enable suppression of ModelExistsWarning from doctests
Merge remote-tracking branch 'pysb/master'