Add MRG Segment.

Create MRG Segment.
It contains below fields.
- prior_patient_id_list
- alter_patient_id
- prior_patient_account_num
- prior_patient_id
- prior_visit_num
- prior_alter_visit_num
- prior_patient_name

I referred from "http://hl7api.sourceforge.net/v25/apidocs/ca/uhn/hl7v2/model/v25/segment/MRG.html"