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."
Properly set owner in SubscriptionsController#create
Subscription#subscription_owner=
Properly set subscription owner model in Koudoku initializer
Fix friendly_id bug.
-bugfix: install_generator makes subscription#credit_card_token accessible
in Rails 3
Add config setting to enable/disable prorated subscriptions
Set default prorating config to true as per Stripe's default
Rails 4 routes install fix
Rails 4 routes drawing update
Update subscription.rb

add subscription owner email try and name || id try for description
Added some syntax highlighting to README
More syntax highlighting
Merge pull request #54 from ZirconCode/master

Rails 4 routes install fix.
Merge pull request #56 from todddickerson/patch-1

Update subscription.rb
Merge pull request #32 from sgruhier/master

add finalize_new_customer!
Merge pull request #50 from hanchang/prorate

Add config setting to enable/disable prorated subscriptions
Merge pull request #62 from samdunne/patch-1

Added some syntax highlighting to README
Merge pull request #39 from strika/master

Fix support for subscription owner other than user
-bugfix: setting customer email address when creating Stripe customer
object