Enhance the TipCalculator with the optionals:

- User can select custom tip percentage if desired
- Experiment with trying input widgets to replace the buttons and/or textviews
Add initial README with animated walkthrough
Change UI layout to support split the tip
Implement user ability to split the tip
Break up onCreate method into few private methods
Fix empty bill amount raising exception
Add tip present and changed the user interface
Merge branch 'extras'