Whitelisting parameters required for successful display of posts.

The move to Rails 4 and strong parameters necessitates whitelisting of
parameters being processed by the controller. The exclusion of the
:tag_list, :published_at_natural and :slug parameters from the
whitelist means that these data are not being saved to the db and hence
no new post will be displayed on the front end, no new tags will be
saved and the post slug cannot be updated after initial creation. This
commit fixes these issues.
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
Merge pull request #90 from gaelian/strong-params-fix

Whitelisting parameters required for successful display of posts.