Fix resizing problem in Firefox

See https://www.mediawiki.org/wiki/Extension_talk:WYSIWYG#Can.27t_resize_images_with_the_cursor_.28Firefox_31.0.29_46995
Fix "size reset" problem

The issue was :
When an image has a defined size, and when you clear the height and width fields, the removal of height and width tags is not remembered (because tags like "_fck_mw_width" are not removed).

(This issue was distinct and don't interfere with https://github.com/Mediawiki-wysiwyg/WYSIWYG-CKeditor/pull/17/files )
Merge pull request #17 from Varlin/patch-9

Fix resizing problem in Firefox