Imporve blog archive portlet

@maethu The blog archive is now collapsible too:

![bildschirmfoto 2014-01-29 um 15 56 52](https://f.cloud.github.com/assets/557005/2030384/10c25f8c-88f6-11e3-8e40-e82fe49ae0b9.png)

@ninfaj will do theming stuff.