Fixes related to correct whitelisting of attributes for comments.

Start making use of the comment_params method in
admin/comments_controller.rb.

Removed author_url and author_email from the params whitelist in
comments_controller.rb as according to the spec, these should not be
able to be set from the front end anyway.
Spec coverage for Rails 4 strong params functionality.

Added factory_girl factories for use with update specs on:

admin/comments_controller_spec.rb
admin/posts_controller_spec.rb