Allow for authoriztion of card if order total is zero

Unless you want to actually make a gateway authorization request for some reason, this seems like it should be a ubercart core issue. It seems like all gateways should be treated the same (skipped).

Okay, so there's a bunch of things going on here:

1) I'm currently working on a 2.x branch which will use uc_recurring. So, I'm not going to add any new features to the old branch (1.x).

2) If you're using the recurring functionality, I don't think you actually need to do an authorization just to validate the credit card. The recurring fee creation includes a card validation already, and the order can be set to fail if the recurring creation fails. This assumption will need testing after the 2.x branch is released.

3) The code provided will need to be smarter / more configurable for me to put it into the official release. Other sites may not want the change. This usually isn't too big a deal (checkbox in settings form, etc.), but needs some thought.

4) I think you might be doing something strange. What kind of a subscription model, etc. are you trying to set up with no up-front fee? It seems like there should be a better way to accomplish the same goal.

The more I think about it, maybe it would be better to just add a hook and let you do whatever you like inside your custom module.

Ah, the coupon throws a wrench in the process. This still might "just work" in 2.x so I'm just going to let it sit for now.