Add "MedicationDispense" Service to Core

In TRUNK-6071 we are adding a new MedicationDispense domain object to core.

This ticket is about adding new MedicationDispenseService to core, following the pattern for other services in Core, that provides some basic API methods:

• getMedicationDispenseByUuid
• getMedicationDispenseByPatient
• getMedicationDispenseByDrugOrder
• saveMedicationDispense
• voidMedicationDispense
• unvoidMedicationDispense
• purgeMedicationDispense

A test class should also be added that tests each of these methods, and some sample MedicationDispense data should be added to the standard test data set

  This can just be added to the master branch of Core, for the 2.6 release.

Further info:

https://talk.openmrs.org/t/proposal-medication-dispense-openmrs-module/36021

https://wiki.openmrs.org/display/projects/Medication+Dispense+Data+Model