Start AddExpenseActivity on edit button press

In case the user presses the edit button in the Show
ExpenseActivity the user must be taken to AddExpenseActivity
with the info about this current expense that the user will
have the ability to change. At the moment the button just
takes the user to AddExpenseActivity without referring
to the chosen Expense

-implement the OnClickListener on the edit button
in ShowExpenseActivity that takes the user to AddExpenseActivity
(empty at the moment)