Add a .gitignore file

I'm so used to my system wide one, I forget to add them
to repositories, sometimes.
Only supply automatic migration options when configuring persistent store for reading
Makes no sense to migrate a store that's being created, right? I also like that this neatly solves the issue of overriding those settings should a subclass try to do so from *its* override of -configurePersistentStore…
Rename -enableAutomaticMigration as +requestsAutomaticMigration