Confirm orders, credit orders

Hi,

This PR adds 2 main feature missing from original spree_paypal_express :

- credit orders using spree flow:
![screen shot 2013-11-20 at 9 13 48 pm](https://f.cloud.github.com/assets/159814/1585799/5f87af80-5220-11e3-99ea-dbd5ef98a793.jpg)
payment method should have 'review' preference to true

- confirm orders: https://github.com/radar/better_spree_paypal_express/issues/34
if payment method review preference is true, we should be able to confirm orders on spree.
it worked for 'Billing' landing page (credit card), not sure it did not work for 'Login' (paypal).

Specs are now green btw, should consider adding travis :beer: !