[GRASS-dev] Add support for Python3

[Pietro]

Dear devs,

Next week I would like to start the merge between my local changes and
trunk to support Python3.
As reported in the ticket: https://trac.osgeo.org/grass/ticket/2708

I've tested the changes with python2.6, python2.7 and python3.4.
So far I've not found regressions in python2.X.

The status quo for python3 is:
- the binding of the C GRASS functions is not working;
- several python modules (~90) are not readable/usable, I've already
started to port them (~20)

My idea for the porting is to gradually introduce changes per steps:
- Monday 27, change only the grass-init and the gunittest;
- If not new problems connected to previous changes raise, Wendesday
29 I commit the grass/script stuff
- If not new problems connected to previous changes raise, I will
start updating the GRASS modules.

In the meanwhile I try to get the ctypes working on PY3.

Are you ok with this schedule?
Other better options?

Let me know

Pietro