Added refresh token capability to make application run self contained (refreshes token automatically)

updated app.js
    - added comments indicating imports, route files, express setup, variables, etc
updated helpers/spotify.js
    - cleaned code (indentation of configuration)
    - added refreshApiToken function (to refresh api tokens from passed REFRESH_TOKEN env variable)
    - update
updated routes/spotify.js
    - added before filter for all spotify routes
        - checks for expired access token, and if expired, calls refresh tokens from spotify helper
        - updated refresh token end point