Feature/image attachments

Contains changes requested in https://github.com/yunojuno/django-trello-webhooks/issues/2, following the given requirements. 
Not sure I'd have gone with this design as it's not very DRY, but I guess that is not the point of these changes anyway. 
I assumed Trello set a mimeType into data.attachment in its requests (https://trello.com/docs/api/card/index.html), so requirements 1) and 2) are not really necessary and are not actually used in the template rendering.