[CON-69] Modify existing AUDIT method APIs so that they track auditing history with user attribution

Subtasks:
- [x] Assign unique uid for each user
- [ ] Add additional param for uid for add and remove methods in all of the Engine stores (i.e. Store, BufferedStore, Limbo, Buffer, etc.)
- [ ] Modify how Write and Revision store things on disk to account for uid

<del>Assign UIDs to users in previous versions of Concourse.</del> (Moved to [CON-78](https://cinchapi.atlassian.net/browse/CON-78))

<del>Modify stored data to have uid field.</del> (Moved to [CON-79](https://cinchapi.atlassian.net/browse/CON-79))