Features/python3 [WIP]

This adds python3 support to libavg.

(aka. Works for me)

As it's quite some change I expect there to be bugs.

To get started:
https://www.libavg.de/site/projects/libavg/wiki/Python3

Please note that:
 - I've changed (imho simplified) the work that registerPlugin needs to do.
 - SubscriberInfo.cpp is now C-API based, not pulling in the methodref python module.
Once dropping 2.x support, the methodref module can be dismissed in favour of https://docs.python.org/3.5/library/weakref.html#weakref.WeakMethod
 - getPixels now uses https://docs.python.org/2/library/functions.html#bytearray and MemoryViews; avoiding lots of string-copies. 


The PR is WIP, because I expect there to be bugs. However, tests all run through with python 2.7.8 and python 3.4.1

Any feedback would be appreciated