Refactor Order#payment_process! to remove many responsibilities from it

Hi folks,

I've been looking in Code Climate's project and take one of "F" class to refactor something. I've cleaned the `Order#payment_process!` to remove too many responsabilities that it did. Also, updated related tests to new RSpec 2.14 / 3 syntax keeping all expectations and stubs properly.

I worked in Spree's `master` branch.