Fix resource issues

Sets of data that demonstrate the issue that was solved per commit:

2b6875b91433640b660734ae026df30b755aabff: Allow null to be the value of an embedded item.

``` php
$noNullValues = array(
	"_links" => array(
		"self" => array("href" => "articles/123")
	),
	"_embedded" => array(
		"topics" => array(
			array(
				"_links" => array(
					"self" => array("href" => "topics/sciencs")
				),
				"name" => "science",
				"slug" => "science"
			),
			array(
				"_links" => array(
					"self" => array("href" => "topics/astronomy")
				),
				"name" => "astronomy",
				"slug" => "astronomy"
			)
		),
		"tags" => null
	)
);
```

2590f00f50a7ae1c07c0bb7f2b7dd87b9900e84: Allow a link element to be an array of links. See 'admin' link element in the spec: http://stateless.co/hal_specification.html#examples

``` php
$multipleLinks = array(
	"_links" => array(
		"self" => array("href" => "articles/123"),
		"related_articles" => array(
			array("href" => "articles/124"),
			array("href" => "articles/125")
		)
	)
);
```

d4a7834db2257ce54a47ffe50cdb1b920e4890c9: Reworked _addData() method to correct a couple problems and simplify. When data consists of nested, associative arrays, the keys that were outputted into XML were incorrect. When a top level element contained an array containing string elements, _addData() failed because it was expected the values of the array to be arrays themselves.

``` php
$addData = array(
	"_links" => array(
		"self" => array("href" => "articles/123"),
		"related_articles" => array(
			array("href" => "articles/124"),
			array("href" => "articles/125")
		)
	),
	"keywords" => array(
		"testOne" => array("space", "astronomy", "orbit"),
		"testTwo" => array("earth", "meteor", "star")
	),
	"nested" => array(
		"moreNested" => array(
			"evenMoreNested" => array("mario", "luigi", "peach")
		),
		"anotherNested" => array("metal man", "wood man", "air man")
	),
	"string" => array("one", "two", "three")
);
```