Updates, Upgrades and Additions

Updated Postfixer to include variable encasing.
x+2, x^2 + 2, 3*x^2 +2are now changed to (x)+2, (x^2) + 2 and (3*x^2) +
2 during input to avoid order of operation confusion

Upgraded Expression and Variable to properly parse variables.

Added Polynomial. Supports univaritve variables and polynomial addition