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). 
In the future, you might want to validate the format of this as being a valid email address with a regex. It doesn't look like you're actually doing any emailing (yet) so it's probably fine for now.