Refactored code to get it working with latest ActiveRecord

I used ar-dbcopy once in the past and it worked fine for me, but when I tried using it again recently I was getting a bunch of errors.  I'm not sure if something changed in ActiveRecord or what, but the only way I could seem to get it to work was to not use SourceDB and TargetDB derived from ActiveRecord::Base.

The changes I made in this branch allowed me to convert from MySQL to Postgres.  Hope this is useful to someone.