Add low res versions of images that were missing

This should be automated in the future. The way that I
imagine it working is:

  1. Grab all images which have the @2x suffix
  2. Filter out all images which have a non-suffixed version in the src
     directory that is newer than the @2x version.
  3. Print a warning for all non-suffixed images that are older than the @2x versions.
  4. Generate a half-sized image with the suffix removed and put it
     in the destination directories. Do not modify anything in the src
     directory.

The benefits are that we still have a way to generate a low-res version manually
but we don't have to when it's not completely necessary.

This will have to be part of the gulp restructuring alluded to elsewhere.
missing images
Merge pull request #50 from PRX/fix/low_res_images

Fix/low res images