Fix order number text

The translation that is currently used expects an argument and
displays 'Order %(number)' without it.  This will now use the same
translation as the admin orders list.
Conditionally add controllers & views into engine path.
Login nav partial belongs in backend.
Dont load_order for admin/orders_controller#new
Generate api key for admin.
Include Spree::Core::UserPaymentSource in user model
Rip out admin users controller in favor of having it in spree backend
Added details on how to use in native apps.

Fixes #168
Reference Rails::Engine::Railties within Engine

Referencing Rails::Application::Railties leads to an exception when running under JRuby

Fixes #169
tab through login elements in the order they appear

Fixes #171