Added the plus button

plus button works for adding 2 or more numbers.
e.g.
1+1+1+1=4
works for decimal too.
adding numbers to the previous answer does not work however
1+1=2
+1=1
i could make a fairly complicated method to amend this or add an ans
button to equate the current number to the last answer to facilitate
this.
I'm getting some rounding errors due to floating point numbers, I
believe bigDecimal or something along those lines will remedy that.