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
in this project, we have included the [shoulda-matchers](https://github.com/thoughtbot/shoulda-matchers) gem which allows us to write expressive statements to test simple validations like these, for example, using the `shoulda-matchers` gem, we could write:

```ruby
it { is_expected.to allow_value('https://www.facebook.com/filename.jpg').for(:image_url) }
```