schema.org and oembed

I added some metadata in the schema.org format. Now we should support really all relevant formats when displaying items.

In oembed I added a small switch so now you can disable the displaying of "rich" oembed data. (Which can be a security problem) Additionally the case was fixed, when a oembed title wasn't present. In that case i the past we displayed the url - which wasn't very beautiful. Now the system looks for other information.