Add option to save images in selected image style

Rerolling the patch in #3 to the 3.0.x dev branch to make it apply to the current 3.0.1 release.

I updated the patch provided in #5 to incorporate the first change suggested in #4 (use image_style_options() to get a list of image style options), I kind of overlooked that completely justified suggestion when rerolling the patch. About the second suggestion made in #4, this feature does lead to code duplication, will think about what is best to do here.

In the mean while, I uploaded the updated patch reflecting the first suggestion made in #4 here.

Correcting small typo (triggering an error) on line 161.

It needs updating the imagecache_external.settings schema.

@claudiu.cristea can you clarify what you mean with "It needs updating the imagecache_external.settings schema."? The current patch already adds imagecache_external_save_as_image_style: '' to the schema.

Re-roll patch in #7 to make it work with version 3.0.3 of the module.