Add support for WITH statement

Postgres 8.4 supports the WITH statement, and in experiments I have found it to be a powerful optimization tool that also can make queries more readable, which is a nice and relatively rare combination. I'd like it to be added to Storm's Select statement support. I'm interested in implementing this if I can find the time. Note that the "recursive" aspects of the WITH statement are less interesting to me right now, but I'd be willing to try to include that too.