Make image work like in html when width xor height != nil :)

[Bluebie, Jenna]

Another microsuggestion: It would be nice if image(), when given a  
width, or a height, but not both, would size the image to that width/ 
height, and size the other dimension automatically to maintain aspect  
ratio. For example:

If I had a png which is 500x1000 pixels big, and I do  
image('bigpony.png', :width => 200), it would size height to 400. :)

Another thing which would be nice is if it were possible to specify  
that image should 'fit within' rather than stretch.. maybe :fit  
=> :stretch default, :fit => :within for aspect ratio maintaining  
behavior where both width and height can be specified as maximums.

P.S. _why, should I just twitter these micro-suggestions? I am way too  
impulsive to save them up for one big email and feel like i'm clogging  
the mailing list with little posts.