- changed "BooleanExpression" to subclass from "BinaryExpression", so that boolean

- changed "BooleanExpression" to subclass from "BinaryExpression", so that boolean
expressions can also follow column-clause behaviors (i.e. label(), etc).
- query.select() had to become more picky about what it considers to be a full "selectable"
and what it considers to be a fragment that represents a WHERE criterion - looks for the presence
of a FromClause now (which is still pretty liberal, since i originally intended the check to be
for select() only).  the previous exception-catch method also added a little stack tracing
overhead anyway.