Removed Retireable from DrugReferenceMap.

Renamed the column drug_reference_map.map_type to drug_reference_map.concept_map_type.

Added equals() and toString() methods to api/src/main/java/org/openmrs/DrugReferenceMap.java.

Added DrugTest.java class.

Removed toString(), equals() and hashCode() methods fromDrugReferenceMap. Improved tests for the Drug class.