Allow the usage of IOStrings and do not close file objects

[robertpenz]

It would be nice that the tftpclient could be used to download a file into memory (IOStrings). I did a quick and dirty method for TFTPClient:

    def getAsIOString(self, filepath):
        """ zieht das file in einen String """
        self.PTFTP_STATE = state.TFTPState(self.peer, proto.OP_RRQ,
                '', filepath, self.transfer_mode, not self.rfc1350)

        self.PTFTP_STATE.file = StringIO.StringIO()
        self.PTFTP_STATE.packetnum = 1
        self.PTFTP_STATE.state = state.STATE_RECV

        opts = dict(self.opts)

        # When not running in RFC1350 compliance mode, append tsize: 0
        # to the list of options in the request to get the requested
        # file size back in the OACK.
        if not self.rfc1350:
            opts[proto.TFTP_OPTION_TSIZE] = 0

        packet = proto.TFTPHelper.createRRQ(filepath, self.transfer_mode, opts)

        self.sock.sendto(packet, self.peer)
        self.handle()

        if self.error:
            error, errmsg = self.error
            raise IOError('Downloading failed (%r, %r)' % (error, errmsg))
        return self.PTFTP_STATE.file

But that only works if I remove the close() functions in state.py. But as I'm not that deep into that code I don't know what that breaks.