Generate User model

*And run migration
Add validations to User model
Add email uniqueness constraint to database

*Add index to email address in database
*Make all email addresses lower case before saving them
Add the bcrypt-ruby gem
Add password_digest column to User
Ensure the user has a secure password

*The password and password_confirmation must match
Add tests for authenticate method for passwords
Add validation for password length
Get rid of i18n warning