Implement new Add Expense button as a footer

As part of the new navigation concept the add expense button
had to be impelemented as a footer in both fragments.

- Change the layouts for both fragments to relative so that the
alignParentBottom could be implemented to the AddExpense footer
button
- Delete the history and the budget buttons as they are not
necessary anymore
- Implement a footer AddExpense button and connect to the
SaveExpenseActivity by onClickListeners
- Upon Saving the Expense - finish the SaveExpenseActivity instead
of going to ExpenseListActivity