Create a basic authentication system using Bcrypt gem

[coolbrg]

https://github.com/codahale/bcrypt-ruby gem provides simple authentication.

For this we need to

• create a User model
• /singup view which will show username and password field to sign up (require signup GET route)
• on clicking signup, signup POST route handle (see account creation step here https://github.com/codahale/bcrypt-ruby#creating-an-account)
• create a /login view which will show login form (require login GET route)
• create a login POST route to authenticate like here (https://github.com/codahale/bcrypt-ruby#authenticating-a-user)
• Only show the 'Cast your vote` button when user is signed.