Add app.json & logo (png)

The app.json file is needed by Heroku for using the 'Deploy to Heroku'
Button.

- Link: https://devcenter.heroku.com/articles/heroku-button
- Issue: heroku/node-js-sample#72

Author: dmfila

app.json

@@ -0,0 +1,7 @@
+{
+  "name": "chat.io",
+  "description": "A Real Time Chat Application built using Node.js, Express, Mongoose, Socket.io, Passport, & Redis.",
+  "repository": "https://github.com/OmarElGabry/chat.io",
+  "logo": "https://raw.githubusercontent.com/OmarElGabry/chat.io/master/public/img/chat.io.PNG",
+  "keywords": ["real-time", "chat", "nodejs", "express", "mongoose", "socketio", "passport", "redis"]
+}