Move Rake tasks to Pliny gem

This moves Rake tasks down into the Pliny gem, freeing up the lib/tasks directory for user-created application logic.

The one disadvantage here is that Rake is now moved into the bundle, and `rake` must now be executed as `bundle exec rake`. This was mostly because requiring the entire bundle was causing `Sinatra::Namespace` to conflict with Rake's ability to define tasks, but long term is probably better practice anyway.