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 tend to stay away from using meta-programming to generate tests, it can lead to tests which are very difficult to debug (because of the incomprehensible stack trace). If the tests are refactored to use `shoulda-matchers`, it would be easy to test invalid URLs.