add finalize_new_customer!

Split customer creation into 3 steps
1) Create customer without plan to get a customer_id
2) call finalize_new_customer! (new callback)
3) Update customer subscription by setting selected plan

It allows to create intermediate invoice items if need be like described in https://support.stripe.com/questions/metered-subscription-billing
"Create any invoice items before your customer is subscribed to the plan, and then update your customer's subscription."
Merge pull request #32 from sgruhier/master

add finalize_new_customer!