Fix typo in Gemfile
Add bcrypt gem for password hashing
Install and configure rspec
Generate User model and migration files
Setup users database table and migrate database
Validate format of usernames and email addresses

Rules for emails are what one would expect.

For usernames, the requirement is that a username must start and end
with a letter or number, but it may contain hyphens and underscores.
Add validations for Users
Add User methods to set and check passwords

Uses BCrypt to hash and salt the password.
Add methods to generate and reset session tokens
Require user session tokens prior to validation
Move user helper methods to private
Add User method to lookup users by credentials

This method - User::find_by_credentials - is flexible enough to take
either a username or an email as input, alongside a password.
Move additional user helper methods to private
Make User::find_by_credentials a class method.

It had accidentally been set as an instance method.
Fix two more bugs in User::find_by_credentials
Generate skeleton controllers for Users & Sessions
Add helper methods for logging users in and out
Define routes required by basic user authorization

Users#show is a temporary placeholder. Ultimately, the apps main page
will replace that.
Fix typo in SessionsHelper
Setup basic landing controller/view for testing

Also removed the boilerplate index.html.