Add support for the Customers resource

The new Customers resource simplifies merchant underwriting and is generally a better abstraction than the Accounts resource. The Accounts resource still exists, but is basically considered deprecated moving forward.

The relevant spec is here: https://github.com/balanced/balanced-api/blob/master/resources/customers.rst
And the original PR that added Customer support to the api is here: https://github.com/balanced/balanced-api/pull/305