added optional serial configuration to demo app
added optional serial configuration to demo app

when launching gsmmodem.py from bash (python -m pygsm.gsmmodem PORT),
the serial port can be configured (since all of ours seem to need it)
by passing key=val pairs after the port.

to launch with my MTCBA-G-U-F4:
python -m pygsm.gsmmodem /dev/ttyUSB0 baudrate=115200 xonxoff=0

rapidsms/pygsm,adammck/pygsm