Added Procfile (for heroku deploy).
Move payment processors into their own model.

(current-processor) provides all that's needed for now.
Moved offers into a model.
Allow tracking current payment processor in the session.

Uses Ring's session for this, currently an in-memory store.

Added sandbox's strawmann account to payment processor list.
Allow setting payment processor from drop down in nav bar.
Code cleanup for payment-processor model.
Clean up auto-submitted payment processor drop-down.
Use encrypted ring cookie for session store.

Becasue We Can, really.  No real need to.
Stay on current page when changing payment processor.

We don't track current page atm, this just uses Referer.
Generate and handle individual return and cancel_return URLs.
Use "action" instead of "a" as query param name.
Show purchase completion status.

Shows the PDT params as obtained from the return url, doesn't fetch
them as yet.
Added fetch-pdt-status.
URL decode PDT response values.

Simplified response parsing.
Display PDT response values.
Cosmetic tweaks to cart template.
Added model for tracking current hostname.

Noitifications  (IPNs) require publically reachable hostnames.  This
model lets you set and track it.
Use new model to calculate notify-return url.
Allow setting current hostname from layout.
Reset to default if an empty hostname is passed in.