Use newer packages

[openpaul]

Currently DNApy can not run using wxpython-gtk3.0 but only with wxpython-gtk2.8.
I propose updating to the new version.

Moreover I suggest moving from python 2.7 to python 3.

I just had the expericene to not be able to launch using Debian 8 and had to downgrade wxpython.

[openpaul]

As I quickly found out, wxpython is not ready for Python 3 yet. So we are stuck with python 2.7 until project phoenix takes off (http://wiki.wxpython.org/ProjectPhoenix).

Moreover it seems like the update to python-wx.gtk3.0 package is possible using python 2.7 but right now it crashes when this library is installed on my system. Changes from 2.8 to 3.0 are listed here: http://docs.wxwidgets.org/3.0/overview_changes_since28.html