Allow json to show one rel as an array instead of an object

According to [The Hal Specification](http://stateless.co/hal_specification.html), "A resource may have multiple links that share the same link relation. ... If you're unsure whether the link should be singular, assume it will be multiple. If you pick singular and find you need to change it, you will need to create a new link relation or face breaking existing clients."

This code change allows you to specify when you add a link that the rel should be forced to an array, even if only one element is available.

```
$hal->addLink( $rel, $url, $properties, $forceArray);
```