User can delete posts

Added a button in the posts edit view, allowing the current user to delete their own posts.
Relabeled "edit" links in the index and show pages to "edit or delete." 
Ahh, you are allowing admins to do this as well. In that case, you are going to have problems with your current code on `edit` and `update` because you are finding the post through the current_user and the admin wont own that post