add pod with discussions from mailing list to the home page

Hi folk,
the mailing list is the most important communication way for our user group. So we need making the mailing list more visible on the onruby page. I made a proposal to implement this.

![bildschirmfoto 2013-08-04 um 11 06 30](https://f.cloud.github.com/assets/419651/906827/3289b2de-fce5-11e2-9dd9-308e06ef9dc5.png)

You must set the "mailing_list_feed_url" setting in the whitelabel.yml for you usergroup to activate the mailing list pod.