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). 
I was actually hoping for some input on this. I was reading that it's better to do email confirmation by sending a confirmation link as there as some weird emails that usually get excluded from the typical email regex. Thoughts?