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
The placeholder text should follow the other placeholders standard and be something like "URL to image" or "Image URL" If not, we should consider changing to "Type your name here" and "Type your description here".