For each resource supplied by the target API create a class here with the necessary methods to access
that resource. For instance, if the target API provides an 'Invoice' resource there should be a 'invoices'
class here to access it. You will also need to add or modify the includes in gingr/client.rb.

Feel free to rename or replace the existing 'customers' and 'transactions' classes. They are only provided
as samples.