Add image support

[Marc MacLeod]

• Entry has_many images.
• Add remove images in the entry editor in the web app.
• Accept image attachment in post by email functionality.
• Store images on S3, most likely using the carrierwave gem.
• Store S3 credentials in the application.yml file.

[Zottelchen]

Would be cool to see this. But please also give an option to save the images on the local server.