Fix DocumentFixture.Serialization failing test on OSX

DocumentFixture.Serialization test in openstudio_utilities_tests fails
on OSX and results in a segmentation fault 11. It is due to
"unregistered class - derived class not registered or exported" which
means the class needs to be exported to boost aka BOOST_CLASS_EXPORT()
Merge pull request #551 from ORNL-BTRIC/Fix_serialization_test_mac

Fix DocumentFixture.Serialization failing test on OSX