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
`ImageUrlValidator` is well tested on it’s own, however it appears you are missing a test that asserts that this validator is being used for the `Project` model.