Got the unit tests running again, apart from the two that were not working in

Got the unit tests running again, apart from the two that were not working in
the first place.  The changes to process_relationships and to sqlachemy itself
were causing a double 'assign_mapper' call to cause issues.  Now, we basically
defer calling assign_mapper until process_relationships in cases where there
are defined relationships.

Also, I moved ActiveMapper to always use the default engine, as there were a
lot of hacks inside ActiveMapper to allow for engine swapping.  The use of the
default engine and the "global_connect" functionality significantly improves
the usability of ActiveMapper.

ActiveMapper will be getting a bit of a refactor/cleanup at some point in the
nearish future, as it has drifted a bit to complexity with the addition of some
features.  For now, this should do the trick!