Add support for data table roles

[PhantomWatson]

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'
));