need full support of math/boolean operators

[Mike Bayer]

Python lets you override pretty much every operator: +, -, *, &, |, etc etc. put support in the SQL package so ClauseElements can use these operators. & = and_, | = or_, ~ = not, etc. also insure that you can make expressions out of columns and literals, i.e. select(+ 10, foo.c.lala + foo.c.hoho)

[Mike Bayer]

the basic idea on this one is committed as of changeset:688.