Enable managing admins and database privileges

Howdy,

This PR adds a few of the API endpoints highlighted under `Security` here : http://influxdb.org/docs/api/http.html, particularly cluster admin management and the ability to (de-)elevate privileges for a user on a database.

Here's where it gets weird ... I can install the gem based on this code and call these methods fine (receive 200, etc), and I can get 200s using curl, but the tests I wrote totally fail. Maybe it's just a weird tunnel vision thing, but I cannot for the life of me figure out what's wrong. More than likely, I just wrote bad tests or there's a weird ordering thing. Here's a gist of the failures : https://gist.github.com/jakedavis/7603715

Anyway, gonna keep debugging, but in the meantime feedback totally welcome. Thanks for review!