Allows setting the serial number of CP2102 USB-TTL bridge dongles.
From the factory, these default to a serial of 0001.  SiLabs provides a
totally usable gui app for setting this serial, and more fields, but
I used this as a basic trainer for working out to read the usb traces from
wireshark to work out the protocol myself.

SiLabs software is at http://www.silabs.com/Support%20Documents/Software/AN721SW.zip

Usage:

$ sudo python write.py "new serial"
INFO:MixedCygnals:Serial successfully changed to new serial
$ sudo lsusb -v -d :ea60 | grep -i serial
  iSerial                 3 new serial
$ sudo python write.py 9876cafe
INFO:MixedCygnals:Serial successfully changed to 9876cafe
$ sudo lsusb -v -d :ea60 | grep -i serial
  iSerial                 3 9876cafe