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). 
Since this has a uniqueness constraint, you might want to look into using a [sequence](https://github.com/thoughtbot/factory_girl/blob/master/GETTING_STARTED.md#sequences)