Add first unit tests for List model

* Add Rspec using `rails generate rspec:install`
* Test associations using `shoulda_matchers`
* Remove test/ dir

Add tests for List's associations using `shoulda_matchers`
Add tests for List's validations
* Rather than create a named scope, use ActiveRecord's helper methods: e.g. `current_user.lists == List.get_users_list(current_user.id)` These are free with all AR Relations

Minor:
* Sort model attrs > associations > validations > scopes
* Clean up `spec_helper` file