- Query.from_self() as well as query.subquery() both disable

- Query.from_self() as well as query.subquery() both disable
the rendering of eager joins inside the subquery produced.
The "disable all eager joins" feature is available publically
via a new query.enable_eagerloads() generative. [ticket:1276]
- Added a rudimental series of set operations to Query that
receive Query objects as arguments, including union(),
union_all(), intersect(), except_(), insertsect_all(),
except_all().  See the API documentation for
Query.union() for examples.
- Fixed bug that prevented Query.join() and eagerloads from
attaching to a query that selected from a union or aliased union.