ENH: reputation system for submissions

1. change existing scipy_central.thumbs.models.Thumbs to use ContentType fields
2. Add views, forms, admin interface for POST request of vote (thumbs)
3. client side code is done using Ajax
4. add Reverse generic relatings, model methods in scipy_central.submission.models.Revision model for reputation system
5. build up-arrow, down-arrow css (used .less, .css is just generated)
ENH: reputation for comments

There is only "upvote" for comments. Downvote is not allowed
reputation onclick event is directly mentioned in list.html