Allow to alter the style associations

Hello,

We should allow to modify the style associations.

The current style association is:

$replace = array(
  'image_crop' => 'image_resize',
  'image_resize' => 'image_resize',
  'image_scale' => 'image_scale',
  'image_scale_and_crop' => 'image_scale',
);

In my opinion, it is not really logical that 'image_scale_and_crop' uses 'image_scale' because the resulted image has absolutely not the same dimensions than an image with a crop operation.

So I've just added a drupal_alter in order to allow developers to choose what style they want to apply if they disagree with the default ones.

Thank you for reading this.