'Retrieve a file from the given path
This method issues the \'RETR\' FTP command.
The file is fed into the given Protocol instance.  The data connection
will be passive if self.passive is set.
@param path: path to file that you wish to receive.
@param protocol: a L{Protocol} instance.
@param offset: offset to start downloading from
@return: L{Deferred}'

def retrieveFile(self, path, protocol, offset=0):

    cmds = [('RETR    ' + self.escapePath(path))]
    if offset:
        cmds.insert(0, ('REST    ' + str(offset)))
    return self.receiveFromConnection(cmds, protocol)
