Major overhaul to the node-backend

[DDuarte]

• Implement a simple HTTP API to open the door and retrieve stats, powered by hapi.js
• Replace the low level RTM Slack module by SlackBots
• Replace some hardcoded SQL queries by an ORM*, powered by Sequelize
• Split the code in multiple files (app, backend and bellServer)
• Slack channel and bot identifiers were replaced by lookups of their names
• The table schema was not changed

The API is as follows:

GET /ping: returns 200

GET /stats: returns the number of door openings per week day and the timestamp of the first door open:

{
    "Sun": 0,
    "Mon": 24,
    "Tue": 26,
    "Wed": 30,
    "Thu": 1,
    "Fri": 0,
    "Sat": 0,
    "since": "2016-03-14T15:26:32.000Z"
}

POST /open: opens the door, params are id: the Slack user id of the person opening the door (must be in the slack channel) and secret: a "password"/token shared by the ones with access to the door. Returns some JSON user params if authorized, 401 otherwise.

The interaction with the bot on Slack was not changed. This was tested with a real slack bot however it was not tested with the doorbell itself.

[luismfonseca]

We've deployed this, tested and worked great.

Although few things need to be changed a bit I'm going to merge this and do the commits later so we can release the new version asap.

Good work :+1:.