do fine-grained copying of __table_args__ based on appropriateness

[Mike Bayer]

continuing from #1393.

[Mike Bayer]

i.e. from parent class to child subclass, some arguments (extensions, mysql_engine) make sense to propagate, others (inherit_condition, constraints) do not. declarative should have a list of attribute names etc. that get passed to child and those that don't.

[Mike Bayer]

don't include mapper_args here. nothing there needs to propagate except that which mapper()/inherits does implicitly.

[Mike Bayer]

we use mixins for this now.

[Mike Bayer]

Removing milestone: 0.6.xx (automated comment)