- improved ability to get the "correct" and most minimal set of primary key

- improved ability to get the "correct" and most minimal set of primary key
  columns from a join, equating foreign keys and otherwise equated columns.
  this is also mostly to help inheritance scenarios formulate the best
  choice of primary key columns.  [ticket:185]
- added 'bind' argument to Sequence.create()/drop(), ColumnDefault.execute()