Added missing fields to OBX segment and improved test

The performing_organization_address and performing_organization_medical_director were both missing from the OBX segment type as you can see by looking at this OBX definition http://www.interfaceware.com/hl7-standard/hl7-segment-OBX.html

I also improved the read test to check all fields of the OBX segment
Correct parse nested children
Merge pull request #9 from calebwoods/fix_obx_segment

Added missing fields to OBX segment and improved test