Revert "Support multiple accounts."

@benwerd I'm going to revert these changes for now as #1 appears to be incomplete (and looking at the code for the twitter/flickr et al plugins it looks very much like you're abandoning multiple account support for free/selfhosted users anyway).

Issues spotted so far:

* [ ] Migration from old to new doesn't work (account page contains a connect link, but doesn't retrieve a login_url from the API (looks like the logic in Account.php hasn't been migrated to support multiple accounts). 
* [ ] No way to disconnect existing account to start again
* [ ] In Main.php the logic for creating the syndication link never fires (detects the presence of the old acc details, so never displays the new ones
* [ ] The post logic appears to only accept the new format syndication, it doesn't test for the old code (like twitter).

Will happily re-merge once things are working! :)