Link Trackers

[cc7768]

I talked with @mmcky a little this morning and we think it would be interesting to implement a link tracker of some sorts to see which notebooks are popular.

Eventually, this could be the basis for the "featured" notebooks in the future -- If we had N featured notebooks then have N-j of them be the most popular notebooks over last 2-3 months and the remaining ones be new contributions (the actual decision of how large to make N and j can come later). This hopefully would allow the notebooks people find most interesting to float to the most viewable positions.