Add tweet_id column to post
Twitter service removes status from twitter when post is revoked
Fix dispatcher_spec to depend on Tumblr which isn't retracting
Service.delete_post now more generic, for future implementations

Service.delete_post now accepts post to be more generic and supports deleting post from any service that overrides delete_post(post)
Merge branch 'feature/retract_tweets_when_post_retracted' of git://github.com/Ruxton/diaspora into develop

Conflicts:
	Changelog.md