TASK-36: create buttons the right way with json skin file

- moved button.atlas, button.png, and "original" files to assets/ui
  - note that an atlas gets associated with a skin
  - therefore, the json file magically knows what
   "ui_background_button_off" maps to a drawable
Merge pull request #52 from nickoram/master

TASK-36: create buttons the right way with json skin file