Choose attribute for configurable product

Added the option to choose on which of the available attributes you want to base your configurable product. For example if you had a product that has size & colour as available configurable attributes and you just wanted to base your related simple products on the colour you would add one (or more) configurable attributes to the additional_attributes like this :
	'additional_attributes' => array(
		array(
			'key' => 'config_attributes',
			'value' => 'some_attribute_code'
		)
	),