small fixes for encoding and signal consistency

Thanks for your great work on yowsup!
I made a small Qt based GUI for the desktop to be able to use it more conveniently:
https://github.com/DorianScholz/yowsup-gui

For it to work better with group and media signals and with non-ascii encodings I had make minor changes to yowsup. Maybe you'd like to incorporate them into yowsup.

And maybe you would also be so kind to add a link to the GUI repo in the yowsup README?