Specify key for sorting blog archive

Specify the year as the key for sorting the blog archive. It might be sorted by the wrong key, for example by count, see screenshot.

![screen shot 2013-11-18 at 14 21 13](https://f.cloud.github.com/assets/5045627/1562918/79780674-5054-11e3-9a7b-b4156dd84c24.png)