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
ENH: order search results based on wilson score

1. New `score` field added to Revision model, which stores wilson, score confidence
2. allvotes_count, upvotes_count model methods - only count when is_valid=True
3. add new field to haystack search_indexes
4. Filter "tag" search based on score
ENH: show top-rated submissions on front page

1. create another tab 'top rated' on front page to show submissions based on score! Filter is created for it
Note: only revision having top Wilson score is showed

2. show all submissions based on score
The above "note" is applicable to [2]

3. create model property in Submission model to get top_revision of the respective submission