Move plos_author migrations to engine

see: https://www.pivotaltracker.com/story/show/81304128

Engine migrations should be initiated by rake xxx:install:migrations. Unfortunately the migration for plos_author was done in the core app. Rather than requiring us to either rollback or drop the db, or ignore migrations, I've backdated the generated migration so that the current migration history remains intact. 
The changes to the `authors` table can live in the core app. It's the creation of `plos_authors_plos_authors` that should be moved into the engine.