Implement alter-user, del-user and del-source

Previously to this, you could add users and sources, but they were effectively immutable because I had not implemented API for changing them. alter-source does not exist because add-source will effectively do what you want. We could add a synonym at the riak-admin level if we want to.

Also, fixes a bug in print-users I found before RICON that didn't make it into the tech preview.