Python 3 support

[Clément Rigaut]

When I try to run the demo script with Python 3.6, I get the following error:

Traceback (most recent call last):
   File "demo.py", line 6, in <module>
     from flat_kivy.flatapp import FlatApp
   File "FlatKivy-master/flat_kivy/flatapp.py", line 6, in <module>
     from flat_kivy.uix.behaviors import LogBehavior
   File "FlatKivy-master/flat_kivy/uix/__init__.py", line 7, in <module>
     from flat_kivy.utils import construct_data_resource
   File "FlatKivy-master/flat_kivy/utils.py", line 4, in <module>
     from color_definitions import colors
 ModuleNotFoundError: No module named 'color_definitions'

I think python3 support is not complete.