Amended the  subtract button

Plus and subtract work well now, fixed a few bugs related to multiple
additions or subtractions in a row such as 1+2-3+4+5=.
Changed how operators are displayed, from appending them to setting an
operator variable, this way only one operator can be displayed.
Changed button43 from His | Gra to Ans | His so I can add an ans
(answer) button, makes current number = last answer.