Tweak app initialization

rename `lib/app` to `initializer`, load everything with that file so callers don't have to manually call `App.initialize!` and load other stuff.

Had to remove `config/initializers/db.rb` and inline it there though. Some ideas to bring it back are to support something like `config/preinitializers`, or to special-case and load that one before models. Thoughts?

Fix #84