fix: allow unicode paths in image source

I am getting this error when trying to use image inside a unicode folder

   File "/home/fruitbat/Documents/kivy/kivy/kivy/core/image/__init__.py", line 202, in populate
     self._nocache, uid)
 UnicodeEncodeError: 'ascii' codec can't encode character u'\xe9' in position 51: ordinal not in range(128)

This PR seems to correct this error. Example code to demo issue here:

    https://gist.github.com/Zen-CODE/81a943a653425fedb216