rgw: Create RESTful endpoint for user and bucket administration.

Expose the following operations through a RESTful endpoint:
    user create
    user modify
    user remove
    subuser create
    subuser modify
    subuser remove
    key create
    key remove
    bucket list
    bucket stats
    bucket link
    bucket unlink
    bucket check
    bucket remove
    remove object

building on the existing /{admin} endpoint.

Signed-off-by caleb miles <caleb.miles@inktank.com>