Allows use of stand-alone Spree Checkout

[MatthewKennedy]

This PR add the ability to use a stand alone Spree Checkout.

1. Checks to see if the app is using the Spree::Checkout
2. Uses routes to the name spaced Spree::Checkout
3. Adds the Login redirect logic to the name spaced Spree::Checkout::OrdersController controller

NOTE: Removes a duplicate check for API in the engine.rb