Always generate width= and height= attributes

[johan]

Since browsers layout web pages faster when they don't have to wait for an image (or fifty) to come down the net pipe, and gravatar_image_tag always knows what sizes they will be, could you add the corresponding width/height attributes to the img tag, too?

Thanks!

[mdeering]

Pushed (1.1.0) with this on by default. Added configuration setting to turn it off to restore the old behaviour.