Add related_urls to category.json

[Daniel-at-github]

Some url are common to the event. Event:

• Web
• Schedule
• Twitter
• Youtube list ...

I think is useful to reflect this in category.json Possible uses:

• Showing this url in every video as event url.
  • This will enable users to compare schedule with pyvideo data and report errors
• Tweet to the event user/hashtag to announce the event is published in pyvideo.org, enabling conference users to review it.