Spree::User => Spree.user_class, plus a fix to the payment page javascri...

...pt

Hi Ted,

I ran into issues due to a client making use of a custom user class.

So, I changed the Spree::User instances to Spree.user_class.

Also, I found an issue w/ the javascript on the payments page.  I believeo the 'please select' warning is added prematurely (on page load), so no matter which referral you select, you always get the warning message.

I'm not sure my javascript workaround is perfect, but it seems to fix the problem.  

I have some limitiations on the current app I'm using that are preventing me from testing all of these changes, but I wanted to provide them to you for perusal.  

Note that you'll also want to cherry-pick into master as well if you agree on these changes.

Cheers!