Added Mixed operations on polynomials.

I added mixed operations on polynomials. Added a real numbers package. Modified `zero-pad` for negative order procedures,
although `adjoin-term` still does not handle them very well.
Modified `type-tag` to work correctly on real and integer numbers. It used to tag them all as "scheme-number".
Added a ChangeLog file, and a list of known bugs to the Readme file.