Fix too generic DOM extractor

* fixes #608
* removal of `<html>` and `<body>` was too generic and failed therefore

* heise feed now works again. Also pictures are back

cc @mohataj @Raydiation 
Maybe consult google/stackoverflow if there is a nice solution, otherwise we have to depend on a newer libxml version which rules out ubuntu 12.04, see http://stackoverflow.com/questions/4879946/domdocument-savehtml-without-html-wrapper