Omniauth facebook without devise

- Added omniauth and omniauth-facebook gem
- created config/initializers/omniauth.rb
- Created signuper app on facebook developer platform
- Added "http://localhost:3000" to signuper facebook app platform
- Inserted facebook JavascriptSDK into index page
- input facebook APP_ID and APP_SECRET in .env file
- .gitignore .env file
- using /auth/facebook to request authentiation
- using /auth/facebook/callback to callback to signuper
Use the `image_tag` helper.