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