Accounts

Hey-o,

Had some downtime over break so I made some additions this this thing. The "models" can probably use some more refactoring to separate concerns.

This PR implements the following:

* **Accounts!** This doesn't affect "payers" much, other than require them to create an account before they can make a payment. For sellers, it enables a whole bunch of stuff.

* **Stripe Settings via Admin** Now if you visit _/admin_ while signed in, you'll see an area where you can enter your Stripe credentials. Technically, anyone with an account can go to _/admin_, enter their stripe credentials, and start using the app to charge people. There really isn't any separation between "Payer" accounts and "Seller" accounts. This might be a little strange, but it's not a security concern. The nice thing about it is that it doesn't _any_ hardcoded credentials in the code.

* **Product Creation** In the Admin section you can create any number of products. After a product is created you can click _Payment Link_ and go to the _Payment Form_ for this product, which you could then email the address of to anyone you wanted to charge.