Change Database Table Creation + (Scheme)

Change Expense table scheme so it will not contain redundant data anymore.

Database connection creation changes so that it uses SqliteDatabaseOpenHelper and instead of creating tables every now and then it is managed by Android itself.

Move Balance and Expense classes to more suitable package called '.lib'.
Add Datasource class for Database usage

Instead of using database manually (opening, closing), it is now using a datasource class which implements open() and close() methods for getting database connectiona and closing database connection respectivily.