don't require billing address

We don't want to require billing_address for #purchase because it
doesn't require one itself in the API.

Added tests to remote for purchases / etc with and without billing
address.

During all of this we also discovered that #store requires a billing
address with at LEAST country and name. From the eWAY folks this is for
legacy API reasons.
Merge pull request #12 from chargify/fix_eway_requirements

don't require billing address