Polling of Inbox / Websockets #143

My considerations:
- We need to create a Pusher account for production and fill the PUSHER_KEY, PUSHER_SECRET and PUSHER_APP_ID variables;
- Each Helpful account will have a different channel on Pusher, the channel string is the account slug;
- The most important piece of code to make this feature work is in Javascript, and it's not tested! I opened a task for it #301
- The empty state of the inbox is loaded with ```display: none``` and Javascript shows it if the async list is loaded empty
- After pusher.js loads a new conversation async the user is not communicated, and the new conversation appears in the bottom of the page, maybe we should warn user that a new conversation was loaded.