Add compatibility with the entity module

To achieve this, we just need to add a 'property_type' key to the color_field_field_info return array with the value of 'color_field_rgb'.

So something like this seems to work:

function color_field_field_info() {
  return array(
    'color_field_rgb' => array(
      'label' => t('Color Field'),
      'description' => t('Field using a hexadecimal notation.'),
      'default_widget' => 'color_field_default_widget',
      'default_formatter' => 'color_field_default_formatter',
      'property_type' => 'color_field_rgb',
    ),
  );
}