Make more clear and user friendly the calibration process

[GrazianoCapelli]

To make more user friendly the calibration process we could:

• Check the position of the phone, in order to avoid to proceed with the calibration with the phone positioned the wrong way. This case we should display a toast that warns the user instead of proceed with the corresponding calibration step.
• Make better pictures to describe the placements. Maybe we could avoid the arrow and use the Android Logo, with a well made phone model.