Adding Flattr buttons

This adds [Flattr buttons](https://flattr.com) to the Geekstammtisch website. There will be two buttons, one for
Geekstammtisch itself and one for each epsiode. Currently the buttons for the show are missing tags and and a
description. The description should be the synopsis of each show but currently it is only available as part of
the `description` element in the feed. Tags aren't available in the feed at all. Both needs to be addressed.

The following has to be solved before a merge is possible:

 - [ ] Find a way to provide a description for each episode
 - [ ] Find a way to provide tags for each episode
 - [ ] Somehow we need to add an icon / image on the Flattr thing
 - [ ] Additionally we should consider using the `rel="payment"` [link in the feed and website][1]

[1]: http://developers.flattr.net/feed/  "Flattr in feeds and HTML"