Added a migration to create the column 'password_digest' for users.

Added the bcrypt gem, to support ActiveModel's has_secure_password.

Added has_secure_password to the User model.

With that, we can create a User instance with an encrypted password, for example from the console:
> User.create first_name:'Alex', last_name:'Deva', password:'111111', password_confirmation:'111111'
 => #<User id: 2, first_name: "Alex", last_name: "Deva", email: nil, tshirt_size: nil, diet: nil, created_at: "2014-11-12 12:01:10", updated_at: "2014-11-12 12:01:10", password_digest: "$2a$10$iUMdmnyEdc8e6gjwaq87HueLZeDh/zi8lAk/iCI5na8...">

And then, we can authenticate the user. Here's an attempt that fails:
 > User.find_by(first_name:'Alex').try(:authenticate, 'abc')
 => false

...and here's one that works:
 > User.find_by(first_name:'Alex').try(:authenticate, '111111')
 => #<User id: 2, first_name: "Alex", last_name: "Deva", email: nil, tshirt_size: nil, diet: nil, created_at: "2014-11-12 12:01:10", updated_at: "2014-11-12 12:01:10", password_digest: "$2a$10$iUMdmnyEdc8e6gjwaq87HueLZeDh/zi8lAk/iCI5na8...">