Small UI redesign on Preferences_MiniMap

- Simplify GUI.
- Message of Opacity only needed on OSX, else hide it usin settings.IS_MAC_OS to simplify UX.
- PEP8 Fixes.
- Python3 Fixes.
- Update Translations.


![temp](https://cloud.githubusercontent.com/assets/1189414/2752465/dbd0c388-c90d-11e3-87dc-6920ee3b7c87.jpg)


Brancho:  fix/preferences_minimap  :earth_americas: