Added ability to attach one image to a listing

* Used paperclip gem -- added listing_image column to listing table
* Modified listing model according to paperclip documentation
* Permitted :listing_image attribute in listings controller
* Button to choose image file appears in new listing and edit listing page
* For now, 300x300px version of attached image appears with listing on dashboard
Added Ruby 1.9 hash syntax to Listing model
Merge pull request #7 from CranAppleJuice/AD-add-images-to-listings

Added ability to attach one image to a listing