Add images to car

[staszek]

User should be able to upload image of its car. Please use dragonfly gem for it.

• Files should be saved to disk. It will not work on heroku, but do not worry about it for now. * Image should not be valid if it is bigger than 200 kilobytes and it does not have image extension.
• You should be able to remove old picture or upload new one when editing.
• Display thumbnail at car show page. It should not be bigger than 400x200 pixels.
• Remember to add tests