Dlc 20

@sourlows Addresses #31, #39; still needs tests for ListingImage resource.
It prevents ListingImage from needing its own controller; you can access the ListingImage resource through the Listing resource. The latter part `:reject_if ...` prevents empty records from being stored.

[Example implementation, section Nested Attributes.](http://sleekd.com/general/adding-multiple-images-to-a-rails-model-with-paperclip/)