Add resource and migration for User model

Creates a migration to build a data table for User data, and configures
resource routing for User.
Update the database with the new user model.
Add bCrypt Ruby gem to handle password encryption
Create user model

Adds a user model, with name, email, and password validations. A user
can author many articles.
Add User controller with standard #new and #create methods
Add an area to display flash messages at the top of the main layout
Add a sign-up form for new users