Allow subquerys in select, fixes #156 -- the fix for this was kind of
neat as it really didn't require much code.  I needed to fix a hook in
the select that checked types on inputs so that instances of SelectQuery
were acceptable.  Beyond that it pretty much took care of itself.  I
realized I needed to be able to allow selectqueries to be aliased, so I
made Query a subclass of Leaf.  We'll see how that goes!