Back to work

Finally able to work on the project again.
Changes:
- Switched to developing using Android Studio. Related build files for
Studio are pushed into the repository, but it should not affect
developing under Eclipse.
- Added function to the additive switch button in the calculator.
- Optimized code for better performance, removing unnecessary boxing of
contents.
- Converted several methods to 'private'. These methods appear to be
only used within the class file and nowhere else.