Support subdomain configuration

Configuring the client by subdomain is required from now.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Merge pull request #1 from recurly/route-api-by-subdomain

Support subdomain configuration
Omit XML element for empty currency lists

Fixes: https://support.recurly.com/tickets/23622
Merge pull request #2 from recurly/empty_currency_list

Omit XML element for empty currency lists