Create a convention for default checkout pane callback names

From the Rules API:

- callbacks: An array which allows to set specific function callbacks for the action. The default for each callback is the actions base appended by '_' and the callback name.

Committed: http://github.com/rszrama/drupalcommerce/commit/e690325c0438e976af2ed990...

Basically, I added the same $checkout_pane->base property that payment methods are using and construct a default array of callback functions using base + callback. This means I had to change some function names to *_form to *_checkout_form... because for some reason the callbacks weren't using the name of the callback. Who knows why? :-/