Payment types preset

I intend to use this module on a future version of my website. However, I needed an extra feature which is presetting allowed payment types such that users can optionally select within a subset of payment types before being redirected to the Paybox site. I used the same mechanism implemented in the core 'uc_credit' module to set the choice as a session variable.

I also fixed a small bug which prevented the 'Back' link to be a link (a missing >)

Here is the patch for review and eventually integration.