Added www.theartrebellion.com to sites in README.
Catches exceptions in thumbnail image.convert
Prevent some warnings during tests
Restructure content architecture docs - lead in with describing how Page/Displayable are built and used.
Move short_url generation code from template tag to model, so we can use it for tweets.
Added TweetableAdminMixin and applied to BlogPostAdmin - used for sending tweets from admin.
Updated docs for twitter integration - document setup for sending tweets plus move to own section.
Added www.engineeredarts.co.uk to sitesin README.
Merge