mod_rest: Restore 'kind' property in JSON-mapped objects The datamapper schema has no 'kind' field, instead handling it as a top-level property of the currently unused <xmpp> element and doing this early dispatch on the top level element name. This puts the field back into the output JSON.

This module adds a command to `prosodyctl` for listing users.

``` sh
prosodyctl mod_listusers
```