Potential Modifications and Improvements
I know not everybody will have a WeMo switch sitting around (or might not want to shell out $50 for one). Another option would be to use this method to hack a $10 remote control outlet:
If you don't mind having your Raspberry Pi wired to your humidifier, it may be possible to wire your raspi directly to the on/off switch on your humidifier. Of course, it would be smart to use a relay to isolate the circuit.
In either case, some slight modifications will be required to the control.py script to make it operate without feedback from a WeMo switch.
In the future I'd like to add some options to the web interface to change the values in control.xml. However, I wouldn't be comfortable doing this without implementing some sort of authentication scheme to prevent malicious changes.
That's all for now. Feel free to ask any questions you may have. I'll try to check back periodically.