Added Lexical class definition.

The Lexical class and its instantiations will be the ones in charge
of the lexicographic analysis of the input. I still need to test
some of the functions, such as what happens when a token is cut off
because that's where the buffer ends and needs to read again to
continue, but overall I think it's going to work (I hope...).
Right now it's only able to recognize identifiers and sum operators
(+|-).