Python 3 compatibility

[florianfesti]

As promised in #35 here a set of patches that make the master branch (more) compatible with Python 3. As I am not using the sequencer there may still be issues with Python3 left. In general the changes are supposed to keep the code working with Python >=2.6 while also making it run with Python 3.

This patch set does not port all changes that have been made to the features/Python3 branch. This patch set breaks the compatibility of the functions in src/util.py basically assuming they are private.

[florianfesti]

The metaclass patch uses the "future" module. This is what trips up the test suite. If adding the external dependency is unacceptable it is possible just just copy the function in or write something similar.

[hiepph]

Try to build again with this pull request, and works great with python3 without this error, ever again:

from containers import *
ImportError: No module named 'containers'

Applaud :clap:

[mgedmin]

I've attempted to complete this work in #130.