Add support for data table roles

Currently, the Google Charts plugin doesn't recognize roles (like 'annotation') for specified columns. This will allow the use of roles like in the following example: 

```
$chart->columns(array(
	'category' => array(
		'type' => 'string',
		'label' => 'Category'
	),
	'score' => array(
		'type' => 'number',
		'label' => 'Score'
	),
	'annotation' => array(
		'type' => 'string', 
		'role' => 'annotation',
		'label' => 'Annotation'
	)
);
$chart->addRow(array(
	'category' => 'BAC After Birthday Party',
	'score' => .240,
	'annotation' => 'Hammered'
));
```