Add comments, javadocs, refactor code.

- Optimized imports.
- Add comments where necessary.
- Delete ExpenseListActivity. This activity was earlier substituted by
  ExpenseFragment.

CreateExpenseActivity:
 - Make createExpense() method private and remove the parameter.

EditExpenseActivity:
 - Make editExpense() method private and remove the parameter.
Budget:

 -delete operations that are not used anymore.

Expense:

 - Remove the getters and setters of unused fields along with fields.

ExpensesDataSource:

 - Remove a method getExpensesFromTimeStamp as it is not needed anymore.