improve scalar=True subqueries

[Mike Bayer]

cant do operations on a subquery:

select([foo](foo), scalar=True) - bindparam('bar').label('hoho')

you have to turn the select into a full ColumnElement:

select([foo](foo), scalar=True).label('foo') - bindparam('bar').label('hoho')

I think perhaps we should, in 0.4, change "scalar=True" to just select(..).scalar(), which returns a ColumnElement wrapper for the select...and deprecate scalar=True. that will clear up the messsiness between scalar/non-scalar selects.

[Mike Bayer]

went with as_scalar(). main work in changeset:3031, changed the name to as_scalar() in changeset:3033 to avoid clashes with existing scalar() method.

[Mike Bayer]

Removing milestone: 0.4.0 (automated comment)