WIP: Introduce riak-cs-admin

RiakCS has various CLI tools and they are expected to increase number of them gradually in future. To avoid increasing the number of them I suggest introducing riak-cs-admin into RiakCS as administration CLI like riak-admin for Riak. Changes on this PR include follows:

- move the implementation of existing tools into riak-cs-admin
-  just in case, leave existing tools as adopter to riak-cs-admin for a while.
- add cluster-info to riak-cs-admin
- add new stats to riak-cs-admin(TODO: how should we deal with current `/stats` API ?)