Fix for allowing unicode string / path in Settings

Bug fixed:
- put utf-8 in string, and validate -> no more crash due to str()
encoding
- put utf-8 in string, validate, close, open the app and edit the value
-> no more weird space due to ascii->utf8 encoding.
- create an unicode directory, and select it with Path. -> no more
crash at validation.
- create an unicode directory, and select it with Path and restart ->
the path is still correct.

Tested on OSX only.

closes #1084
Merge pull request #1228 from kivy/fix-1084

Fix for allowing unicode string / path in Settings