Look for assets using url_for before falling back to the "images" directory

This makes assets look for resources relative to the current resources when you link to an image, rather than going straight to the "images" folder. This means if you write `image_tag "myimage.png"` and `myimage.png` is in the same directory as your template, you'll link to that, instead of `images/myimage.png` (which may not even exist). This change seems pretty straightforward, but doesn't have any tests (I'm bad).