nsITransfer Interface

 Initializes the transfer with certain properties.  This function must
 be called prior to accessing any properties on this interface.

 @param aSource The source URI of the transfer. Must not be null.

 @param aTarget The target URI of the transfer. Must not be null.

 @param aDisplayName The user-readable description of the transfer.
                     Can be empty.

 @param aMIMEInfo The MIME info associated with the target,
                  including MIME type and helper app when appropriate.
                  This parameter is optional.

 @param startTime Time when the download started (ie, when the first
                  response from the server was received)
                  XXX presumably wbp and exthandler do this differently

 @param aTempFile The location of a temporary file; i.e. a file in which
                  the received data will be stored, but which is not
                  equal to the target file. (will be moved to the real
                  target by the caller, when the download is finished)
                  May be null.

 @param aCancelable An object that can be used to abort the download.
                    Must not be null.
                    Implementations are expected to hold a strong
                    reference to this object until the download is
                    finished, at which point they should release the
                    reference.