WIP Client can create subscriptions intervals.

* Add the new subscription interval form in the layout of cart_form.
* Select the subscription interval after the client created it.
* Refactoring the creation of the new subscription interval controller
Merge branch 'master' into client_new_subscription

Conflicts:
	app/overrides/spree/products/_cart_form/subscription_interval_dropdown.html.erb.deface
Merge pull request #6 from mumoc/client_new_subscription

Client new subscription