Autoload issue with ImpressionistController

Instead of depending on autoload to bring in the
`ImpressionistController` constant, we manually bring it with a call
to `require` and just move that file to `lib`.

Prior to this, Rails would emit a warning on startup about needing to
use autoload durning an initializer like this, letting us know that a
future version of Rails will error from this.

This should hopefully remove that error and still allow these
controller mixins to be available.

lib/impressionist/engine.rb

@@ -9,6 +9,7 @@ class Engine < ::Rails::Engine
 
 
   initializer 'impressionist.controller' do
+    require "impressionist/controllers/impressionist_controller"
     require "impressionist/controllers/mongoid/impressionist_controller" if orm == :mongoid.to_s
 
     ActiveSupport.on_load(:action_controller) do