ENH: add django.contrib.comments to project
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