Add allergies to REST resources

As a client of OpenMRS REST API, I want to be able to create, read, update, and delete allergies.

Allergies are currently implemented in the allergyapi module (though we will eventually incorporate this code in openmrs-core).

This REST resource should be made available as if it were a core resource (i.e. it should not be under an allergyapi namespace).

e.g. GET .../ws/rest/v1/allergies