- merge -r6823:6841 of branches/chrisw_mixin

- merge -r6823:6841 of branches/chrisw_mixin
- declarative now accepts mixin classes directly, as a means
to provide common functional and column-based elements on
all subclasses, as well as a means to propagate a fixed
set of __table_args__ or __mapper_args__ to subclasses.
For custom combinations of __table_args__/__mapper_args__ from
an inherited mixin to local, descriptors can now be used.
New details are all up in the Declarative documentation.
Thanks to Chris Withers for putting up with my strife
on this. [ticket:1707]