Add model.image field (cf #5)

This only cover the needs for a "call" image, i.e. an image
to be used mainly to illustrate the article in a list, and we
still need to manage images in the text, later.
Test that we can create a blog content without image
Merge pull request #55 from ideas-box/add-blog-image

Add content.image field (cf #5)
Add a "dummy data" command to fill in the project with content, in
debug mode