Add support for data formatters

Usage: When defining a column, a NumberFormat pattern can be assigned to
the 'format' key.

Example:
$this->chart->columns(array(
    'category' => array(
       	'label' => 'Timespan', 
       	'type' => 'string'
	),
    'value' => array(
       	'label' => 'Values, 
       	'type' => 'number',
       	'format' => '0.00%'
    )
);