Add the API client side for backups.

Note that backups has its own facade, so it isn't quite as simple as adding a new method on Client.
The first thing I notice here is that BackupsClient is composed out of a Client directly -- while all the other API client "aspects" seem to exist in an api subpackage, and follow the form:

type Client struct {
  base.ClientFacade
  facade base.FacadeCaller
}

Why is the backups client handled differently?