Add change log links to gem page

This adds a new changelog column to the linksets table and displays
that change log alongside the existing linkset links.

![image](https://cloud.githubusercontent.com/assets/65950/4921602/70870fa2-650b-11e4-9488-493b648cb7e0.png)

This is a draft, there may be tests we haven't thought of updating with @jaymcgavren but we will if necessary.

Fixes #578