Look up a Tpf.Persona by its TelepathyGLib.Contact.
If the TelepathyGLib.Account for the contact's TelepathyGLib.Connection is 
              null, or if a Tpf.PersonaStore can't be found for that account, null
             will be returned. Otherwise, if a Tpf.Persona already exists for the given contact, 
            that will be returned; if one doesn't exist a new one will be created and returned. In this case, the 
            Tpf.Persona will be added to the PersonaStore
             associated with the account, and will be removed when contact is destroyed.
| contact | the Telepathy contact of the persona | 
| the persona associated with the contact, or  | 
0.6.6