This is node-style callbacks for #676. This allows us to release
the new callback style without introducing backwards incompatible
changes (or making changes to the binary from 2.0.1). If a node style
callback is provided, it will be used. For now, this stacks with
the original `options.success` and `options.error`, although it can
be trivially changed to an xor.

To highlight the new interface, a separate `describe` was added to the
tests which identifies the `{options}, cb` interface and tests it
independently.