added hyperlink tests for layout common links
added tests for application_helper
added User model
ran db migration to add User model
added attr tests for User model
added validation for presence of user name and email
added validation for max length of user name
validation of email format
ensured email validation uniqueness is case-insensitive
added unique index on email attr in users table and downcase callback for before_save
added password_digest column to users and also password authentication and confirmation matching