rgw_user: Create RGWUser class.

Clean up the existing user operations in order to support a RESTful user
management api.

Signed-off-by: caleb miles <caleb.miles@inktank.com>
rgw_rest_user: Create RESTful interface for administrative user operations.

Allow for user creation, modification, and removal through the
Admin API. Also exposes a user's access keys, subuers, and admin
capabilities as subresources to the RESTful /{admin}/user endpoint.

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