Added some files related to the incoming concept of Expression.
Changed interface IExpression.
Implemented every function of Parser as IExpression.

Now must directly concentrate on parsing.
Added extraction of operators and numerical constants.

Uh, it's possible to use Parser for complete calculation right now.
Changed calling style: now main personally provides calculator with expression.
Fixed typo.
Rewrited Rutishauser's algorithm.
Fixed mistake.
Rewrited algorithm of B'n'Z.
Added method pasteSymbol to interface IExpression.
Rewrited polish stuff.
Feels like I made it work with numerical constants.

Get ready for the next round.
Added variables.