Image url

Created a field in the database, associated forms and views, spec tests, and validator in lib/validators that allow a user to include an image with their project description by linking to a file hosted on a whitelisted website.

https://www.pivotaltracker.com/story/show/80235250
I think a more appropriate name for this method would be `is_domain_whitelisted?` or `is_image_host_whitelisted?`, since it better describes the current functionality