Handle primary_key array (compositive_foreign_key)

Ripped from an old Lighthouse ticket: http://zdennis.lighthouseapp.com/projects/14379/tickets/30-patch-to-support-importing-tables-with-composite-primary-keys

I'll do some further testing and try to even add a test case for this when I get a chance. Putting it out there for now for discussion.

This may fix #111.