encapsulate manual ActiveRecord query in new #after_date scope.

The highly-coupled mock demanded it, and let's update to ActiveRecord 3+
syntax, while we're at it.
update more queries to ActiveRecord 3+ syntax.
update migrations to ActiveRecord 3+ query syntax.

some incidental changes here to get the migrations able to roll all the
way back to zero and forward again.
turns out empty migration down methods are unnecessary.
Merge pull request #80 from botandrose/update_active_record_queries

Update ActiveRecord queries to newer syntax