Rename service methods and read only results.

Makes calling methods on services more natural:
paymill.Clients.GetAsync() vs paymill.Clients.GetClientsAsync().

Also, the return value is now a IReadOnlyCollection<T>, so the intent is
more readily apparent.

This also seems to lay the groundwork for moving more of the
functionality in AbstractService.
Move more functionality into AbstractService.

The individual service implementations now do much less work, following
the DRY principle.

Four interfaces, corresponding to the individual CRUD operations have
been created. Then the combinations of those interfaces that we use
(e.g. full CRUD, only create and read) are exposed on the Paymill class.

The services are no longer public, so externally you can only work with
the interfaces.