Improvement to keyboard app - selecting different language general functions

* Issue: #752
* Changes: (apps/keyboard/app.py)
  - Add language code mapping
  - Add enable_keyboard_layout() function
  - Adding switch_keyboard_language() function
  - Fixing PEP8 problem

* Remark:
I will start to modify following file since the part of function of app.py can be applied to it.
gaiatest/tests/settings/test_settings_change_keyboard_language.py

Thanks!