Resize and create new image based on original aspect ratio rather then overlay over fixed aspect ratio.

Previously when an image is resized, a new image is created with a fixed
aspect ratio based off of the admin panel media settings. If the orginal
image aspect ratio doesn't fit the new image aspect ratio, the new image
background (white in this case) is seen around the edges. This change
allows flexibility with use of css to control the layout dimensions of
the image.