Fixed bug in Postfix notation

The current token was not being repushed to the stack, so tokens were
being lost.
Resolved issue with Prefix function
Implemented expression evaluation

Evaluates integer expressions. Verified with 2+3, 2+3*4, (2+3)*4
Implemented variable replacement

implemented variable replacement for expressions like
a+b-c*d/e;a=1;b=2;c=3;d=4;e=5; Need to allow for multiple evaluation of
expressions such as 2+3; 4+5; and it ouput the results