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 is currently 'Paste a link to your image here'. I think this is the best choice.

For most websites, this would be the opportunity to upload a picture. We need to call the user's attention to the fact that we expect them to do something different. Given that use of the image url field is not as obvious as the name and description fields, I think an explanation of what to do instead of a boilerplate description is the best choice. 

But I will change it if you tell me to. Are you telling me to?