Ability to retrieve Patient ID from OpenMRS using WS

To create a Patient record using REST API, one of the fields that needs to be passed into the API call is the "Patient Identifier". This new WS will get next Patient Identifier that can be used to create Patient records programmatically. The advantage of using the service is, the Patient id will be in sequence whether the Patient is created using OpenMRS UI or thro' Rest.

PS. the service has been created, if its ok with the team, I can commit the change.