Implemented bureau authentication

* Token member for bureaus
* Exception type for failed authentication
* Authenticate function
* Bureau id to credentials
* Fixed bad constructor and toString


git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@5149 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/as/com/threerings/bureau/client/BureauClient.as

@@ -39,13 +39,10 @@ public class BureauClient extends Client
     public function BureauClient (token :String, bureauId :String)
     {
         super(null);
-
         _bureauId = bureauId;
-
         var creds :BureauCredentials = new BureauCredentials(_bureauId);
         creds.sessionToken = token;
         _creds = creds;
-
         _ctx = createContext();
         _director = createDirector();
     }