First ports to python 3

[michi7x7]

Back in January I started using python-deltasigma with Python 3. I will probably continue to use it, but I don't have the ressources and time to test everyhing.

This is by no means a complete Pull-Request, it is just the bare minimum of changes to make some of the examples work in Python 3.

What needs to be done:

• Integer division must use // instead of /
• Consequently, all files must include from __future__ import division for backward-compatibility
• Data-Types are much more strict. This results in quite some int(ceil(x)) or whatever
• There have been some changes to scipy.signal, conversation between TF, ZPK and LTI must be done explicitly.
• I remember having some problems with matplotlib code