Enable querying a list of users using the API.

Example:
 api.php?action=get&what=users

Returns:
{"status":"done","username":"chris","userid":"5","timestamp":"2013-01-23 11:01:24","execution_time":"0 seconds","action":"get","what":"users","id":"*","parameters":null,"result":["nicolas","daniel","rudolph","ellen","chris","dehaes","bahtam","arne","jurriaan"]}