Fix IntegrityError bug.

This bug has been lurking since last week and was introduced in #70.
The effect is that messages get inserted just fine, but their relationships
never make it into place in some cases.  With postgres, we have to force the
insert of the User and/or Package first before we can try to insert in the
association table.  When we used the ORM by itself, this was handled for us.
Now that we drop down a layer to the sql abstraction, we have to make sure our
``.commit()`` ordering is right.