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
This match can still be easily fooled, for example, this regex would allow `http://dangerous.site/evilfiles/bluemelon.com.png` to go through. Ideally you want to validate the start of the string up to the end of the domain, something like: `/\Ahttps?\:\/\/#{…}/` would do better.