Change to core/skin.js to enable setting both background pos_horz and pos_vert values for icon images

As discussed at:

http://ckeditor.com/forums/CKEditor/Considering-making-a-change-via-GitHub-wanted-sanity-check...

Made a few changes to:
CKEDITOR.skins.addIcon() added a fourth parameter, and renamed the
third.  These parameters:
offset_vert[3:Number]
offset_horz[4:Number]
...allow defining a specific icon image file, and allows specifying the
background-position offset for both x and y positions.  I also removed
the "check" not to override any existing value, as a call to addIcon
should always allow overriding a previous value.

I also modified the call to: CKEDITOR.skin.getIconStyle() such that the
background-position is now populated with any pos_horz pos_vert values
defined with a call to addIcon.

Specifying the offset_horz after the offset_vert breaks the normal x,y
nomenclature but it maintains backwards compatiblity of all plugins that
currently only specity an offset_vert value.

My first commit, but reach out at zac@zacwolf.com if you have any
questions/concerns.