First unit test for Echelon Config!
Created engine adapter, db facade, and started migrating migrator
Updating uses of $db in Migrator
Extracted Applied_Versions class hierarchy

Migrating_Forward and Rolling_Backward are kind of like conditional
classes that abstract the note taking when a migration is (un)applied.

Applied_Versions deals with the intricacies of dealing with the database
schema table. I left the init_schema_table() method on Migrator because
the Migrator is the main entry point for the user.