ENH: basic comment system using django.contrib.comments

Comments attached to scipy_central.submission.models.Revision
because we want comments for individual revision of a Submission

Changes Made:
1. add scipy_central.comments in INSTALLED_APPS, add COMMENTS_APPS
variable
2. new field in scipy_central.submission.models.Revision to enalble
Comment Moderation
3. override built-in comment forms
django.contrib.comments.forms.CommentForm to hide all fields except
"comment" --all hidden values are taken by default if signed in
4. Comment form availabe only to signed-in users.
5. comments template is mentioned in list.html
ENH: support restructured text in comments

1. created extra model field rest_comment --which contains restructured text
2. Admin interface for custom comment model
ENH: move comment Form into form.html
ENH: comment flagging feature using ajax

1. Comment Flags are done with Ajax calls, bootstrap Modal
2. After flag is received, 'CommentModerators' Groups receives email
ENH: post comments using ajax

1. django.contrib.comments.views.comments.post_comment() is overridden
2. post comment using jquery forms

Features:
1. Asynchronous posting, preview, loading comments

Features not present:
1. form validation on client side
ENH: Rss Feeds for comments

1. /comments/ latest 40 feeds (Rss)
2. /comments/item_id/rev_id/ all feeds under a revision (Rss)
3. Place feed urls in right-sidebar

Note:
import scipy_central.submission module again for ContentType object app_name