Support for single embedded resource

Hi,
According to the IETF spec for HAL version 5, a single resource should be embeddable cf. http://tools.ietf.org/html/draft-kelly-json-hal-05#section-4.1.2.

Here's an update to your gem so that embedded accepts a single resource.

Cheers,
Julien & Étienne (@etiennebarrie)