Added PAL interpreter.
Implemented pushing global variable to stack.
Modified expression parser to implement PAL code generation for expressions.
Before switch to using inheritance for tokenClass.
Implemented expression evaluation limited to int variables and addition.
Implemented variable assignment and cout.
Added expression evaluation.
Implemented swap operation.
Refactored code.
Removed some magic numbers.
Combined duplicated code.
Added comment.
Changed file input and output back from debugging mode.