Security

Added two new permissions:

* yokozuna.search
* yokozuna.admin

The first permission is read access for an index. Currently it does not support types, only bucket name which represent an index. After speaking with @Vagabond I'm going to expand this to support a default type `index`, so you could grant a user search access like this:

* `riak-admin security grant yokozuna.search ON index indexName TO userName`

We may wish to make this a reserved word for bucket types.

The admin permissions are universal per type index or schema. Meaning, you have access to index, you can read, list, create and delete them. For example:

* `riak-admin security grant yokozuna.admin ON index TO userName`
* `riak-admin security grant yokozuna.admin ON schema TO userName`

We may find a need to break these up later into more fine-grained permissions, but that seems a bit overzealous at this point in time, because it would potentially turn yokozuna's 3 permissions into 7.