Add plugin information block

G'day, this adds the basic plugin information block. It helps identify *which* Memcache(d) object cache is installed on a given website, and provides links to the author's website and the GitHub project. vis:

![drop-ins](https://cloud.githubusercontent.com/assets/1294614/5039284/fca55daa-6bef-11e4-8aef-569213d9df5c.png)