Show only posts from subscribed blogs for currently logged-in user on dashboard

* Install nifty-generators gem into development group
* Create #blog-toolbar with follow/unfollow button, place in top-right corner of blog layout
* Create follow_button helper that toggles user's subscribed state to a given blog
* Require authentication for posts controller except index and show
* Create subscription model/controller
* Create blog_url helper to link to subdomain version of blogs
* Create subscription listing page at /following
* Let cookies persist between subdomains