Implemented following User Stories:
•User is displayed the tip of specified percentage for specified entered amount
•User enters the total amount of the transaction
•User can select between tip amounts (i.e 10%, 15%, 20%)
•Upon selecting tip amount, formatted tip value is displayed
• (Optional) User can select custom tip percentage if desired

Signed-off-by: Cindy Tianqing Li <cindyltq@hotmail.com>