Python 2 compatibility fixes

I made some changes to allow the code to run on both python 2 (tested 2.7.3) and python 3 (tested 3.2.3). Highlights are:

- changes to "super" syntax: from super() to super(Class, self)
- fixed xpub subscription unicode message parsing bug
- added python 2 input fix for observing_client 
- fixed misscapitalization of Tkinter