Move core to frontend

Models will soon become the new 'core'

rename models to core
Move orders controller extension spec into frontend
Fix upload path in product_spec
Depend on frontend gem in dash's Gemfile
Add routes to frontend
Dash needs API within Gemfile as well
Fix dash's spec_helper
[frontend] Add correct dependencies for core + api
Don't pass argument to PaymentMethod.available within Admin::PaymentsController

Related to #1981
Bump README Rails version to 3.2.9
Merge promo into core+frontend
Support money 5.1.0
[frontend] Remove useless comments in Admin::OverviewController
[frontend] use correct flash messages in Admin::ProductsController
Re-add RespondWith module
Remove money dependency from frontend

This exists in core, so no need to have it here
[frontend] Remove display heading check in spec/requests/admin/configuration/payment_methods_spec.rb
Adding Order#variants

Convenience method for grabbing a list of variants

Fixes #2195
Set flash[:success] correctly in Admin::ProductsController
Remove require for spree/core/responder and store_helper in frontend.rb