Fix failing remote test.
Initial commit of eWay Rapid 3.1 changes.

I've rewritten this to use the new JSON API.  I also removed the sandbox
credentials from the shared fixtures.yml since this was tied to a
specific eWay sandbox account and these credentials do not support the
new 3.1 direct transactions.
Finish eway rapid 3.1 gateway.

Finish refund implementation.  Add unit tests. Update rdocs.
Mark the eway direct credentials as invalid.
Merge pull request #1176 from Shopify/eway_rapid_3.1

Eway rapid 3.1