remove ghost.settings and ghost.notifications

covers 90% of #755
- moved ghost.settings to api.settings
- moved ghost.notifications to api.notifications
- split up api/index.js to notifications.js, posts.js, settings.js,
tags.js and users.js
- added instance.globals as temp workaround for blogglobals (Known
issue: blog title and blog description are updated after restart only)
- added webroot to config() to remove `var root = ...`
- changed `e` and `url` helper to async
- updated tests