Add user model and tests

Starting the model of users. Taking advantage of the [`has_secure_password`](http://api.rubyonrails.org/classes/ActiveModel/SecurePassword/ClassMethods.html#method-i-has_secure_password) method built into ActiveModel (requires bcrypt gem). 
You're in luck -- there's a `case_sensitive` option you can pass to the `uniqueness` validation (i.e. `uniqueness: { case_sensitive: false }`).  [Read more](http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_uniqueness_of)