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). 
> Do you use a separate sequence for each field?

You can if you want, but it's most useful for fields with uniqueness constraints.  This is so you don't run into validation errors when creating more than one user in your tests.