Post URLs

We now keep track of a post's post number, and a thread's latest post number. On the client, `/t/:slug/:id/:post_number` will link directly to a post. Additionally, instead of keeping track of timestamps to determine whether a person has visited a thread, we keep track of the last post number they read. This way, when linking to a thread that's unread, we can link to the next unread post for that user instead of going right to the top of the thread.

#144 will need to be changed at some point (depending on the order in which these are merged) to take these changes into consideration.
Perhaps this would be a bit cleaner if post had a `mark_as_visited_for` method as well:

```ruby
class Post
  def mark_as_visited_for(user)
    thread.mark_as_visited_for(user, self)
  end
end
```