patch for scaling to fixed width or fixed height

Issue Summary

For a website i'm working on right now i need images that are all scaled to a fixed width of 150px (preserving aspect ratio).
So here is a small patch to add this functionality.

Explanation:

In the 'admin/settings/webfm_images' interface is no need anymore to enter both values (width and height).
If you enter height only, or width only the images will be scaled to fixed height or fixed width as entered, preserving the aspect ratio of the original image.

I've added some explanation text to the 'admin/settings/webfm_images' interface.

Also added a 'summary'-attribute to the $output = theme('table') in function theme_webfm_images_settings_sizes_form to make the html-output XHTML 1.0 Strict conform.