Common tagging mechanism for openmrs objects

Tagging in openmrs should be handled in a generic way for all openmrs objects in the API.

• We want predictably similar APIs for tagging in different parts of the API
• We will probably want a way to look for tags across objects in the future (e.g., get all concepts, patients, encounters, etc. that have to do with "diabetes")

See Common or Centralized Approach to Tagging (Design Page) in the wiki.