Replace __local_mapper/table_args__ with __merge_mapper/table_args__.

[seth-p]

This is my attempt at fixing the logic for merging __mapper/table_args__ from base classes.

In the absence of callables, the logic for setting cls.__mapper/table_args__ is straightforward:

• if cls.__dict__ specifies '__mapper/table_args__', then simply use that;
• otherwise, merge cls.__dict__['__merge_mapper/table_args__'] (if any) with the recursive calculation of __mapper/table_args__ for every base base in cls.__bases__.

In the case that any of the base __mapper/table_args__ are constructed from callables (e.g. classmethods), such callables are reevaluated for cls.

[dgilland]

Merged #18 instead of this one.