Add parent field to Post

[BoPeng]

NOTE: This is a proof-of-concept PR and will be improved if it is determined to be acceptable.

#227

django-machina currently does not support tree-like reply structure like django-threadedcomments. Where it is difficult to provide full support for threaded comments in templates, it is fairly easy to add a parent field that allows users to track such information, and implement the tree-like display of posts by themselves. This PR would provide the core data structure so that they do not have to override Post.

Note that this PR does not change forum_conversation.urls, and only add ?parent=pk to the post_create url.