Python XMODEM protocol implementation

This is a literal implementation of XMODEM.TXT, XMODEM1K.TXT and
XMODMCRC.TXT, support for YMODEM and ZMODEM is pending. YMODEM should
be fairly easy to implement as it is a hack on top of the XMODEM
protocol using sequence bytes ``0x00`` for sending file names (and some
meta data).