Some (optional) changes

[gemantzu]

• Either remove the logo link, or make it point somewhere to the app
• change Readme.md's last line session/new url to show just that (localhost:4000/session/new instead of localhost:4000). Also, it might be helpful to do something with root route (maybe point the user to the new session if they are not logged in, or make the chats route as the root directory.
• Maybe merge this with master?
• ~empty lib/phat_web/presence.ex and let user type the tutorial in~ Scratch that, I see what you did in the end.

[doomspork]

@gemantzu in the interest of helping @SophieDeBenedetto get her blog out, would you be open to a PR for your suggested changes?