Remove the `app` gem for now

Creperie doesn't really need its own configuration level yet, and Crepe
is handling its own API-level configuration fine for the time being.

Signed-off-by: David Celis <me@davidcel.is>
Bundle jsonite by default

Signed-off-by: David Celis <me@davidcel.is>
Remove extant references to App

Signed-off-by: David Celis <me@davidcel.is>
Move the main API to config/application.rb

Signed-off-by: David Celis <me@davidcel.is>