topic popularity system.

unfortunately, some references to TopicPopInfo model are broken. maybe circular reference to it in topicSchema?
circular reference gone. fixed incorrect warn update on topicPopInfo method name
clicking a topic preview directs to view topic link in preview
load more topic previews on scroll to bottom of topic list
Merge pull request #14 from jackrr/topic-sorting-and-preview-paging

Topic sorting and preview paging