Support other payment methods (Mt. Gox / Bitcoin)

It seems that Amazon Money transfer services are currently only available in the United States.
I've added support for Bitcoin (off by default).

This pull request contains:
1. [Small code changes to make the payment method generic](https://github.com/commerce-sciences/selfstarter/commit/ffc4268c3e8f29306eb414a0132c5a68b6b6c5c6)
2. Adding Mt. Gox as a payment method

To enable it, you need to:

0. Understand what Bitcoins are (http://bitcoin.org/ and http://weusecoins.com/ are a good start)
1. Register for an Mt. Gox merchant account at http://mtgox.com/
2. Create a Checkout Button via Mt. Gox
3. Copy the Payment URL into preorder_controller
4. Uncomment the Mt. Gox submit button in app/views/preorder/checkout.html.erb