Use select_entity_from for SQLAlchemy 0.9+ compatibility

SQLAlchemy 0.9 changed the behavior of Query.select_from(), and this
causes bedup memory usage to explode.  Starting in SQLAlchemy 0.8.2
there is a Query.select_entity_from() which preserves the old behavior,
and this tests fine with bedup on both old and new SQLAlchemy.

Fixes #42.