Only hide 'Save and Continue' button if SpreePayPalExpress.paymentMethodID is set
Show an error for when we cannot connect to PayPal
Fix issue where a default selection of the paypal method would not disable the save and continue button
Remove duplicate install instructions from README
Support $0 item amounts

Paypal Express does not support having zero cost items, or zero cost
item total. This is handled in two ways.

1. If there are some non-zero items, they are displayed through paypal
and the others are ignored.

2. If the items total is zero cost (not including shipping/taxes)
paypal is not given a detailed order breakdown, which results in it
simply displaying "Current order".

Fixes #10
Merge pull request #12 from freerunningtechnologies/support-0-dollar-amounts

Support zero-cost items and item totals
Pass correct amount to build_do_express_checkout_payment

Fixes #11
Store transaction id on source when a payment is successfully complete
Disable the use of the PayPal payment method in the admin backend
Only disable things if SpreePaypalExpress.paymentMethodID is found

This was causing the submit button on other forms to become disabled unexpectedly
Add ability to refund payments from the admin backend