Resize does not work properly, if width/height attribute are not set on the image tag

Currently resizing does not work properly, if the image tag does not have a width and/or height attribute: up-scaling doesn't work at all (if it was downscaled before), because the script has no information about the original width/height of the image and downscaling does not work properly, if you specified only one of the attributes (height / width). 

My pull-request proposes a fix for this: the attributes width/height are set with the value from offsetWidth/offsetHeight, only if the attributes are not already available. so the attributes get set only for (new) images, that have not already a width/height attribute set.

Nevertheless: great work, your rowGrid.js!