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~~
 - [ ] ~~Find a way to provide tags for each episode~~
 - [x] Somehow we need to add an icon / image on the Flattr thing
 - [x] Additionally we should consider using the `rel="payment"` [link in the feed and website][1]

**UPDATE** Per episode Flattr button will be integrated after we fixed the `itunes:summary` element in our feed.

[1]: http://developers.flattr.net/feed/  "Flattr in feeds and HTML"