Paypal Express gateway: Allow an amount of 0

No longer default the amount to a dollar.  There are times when an
amount of 0 is perfectly fine for a Paypal Express transaction.

In fact, an amount of 0 is required if you'd like to do an authorization
without an immediate charge.

https://www.x.com/developers/paypal/documentation-tools/express-checkout/integration-guide/ECReferenceTxns

Closes #655.