shindig atom API enhancements

We identified a few issues of the atom Social API implementation in Shindig:

• The <link> url is output with the url as the content of the tag as opposed to a href attribute
• The url chosen is the url of the event (an API URL) as opposed to the URL of the Object in the event
• The url type is "self" (which is correct for 2. above, but largely useless) when it should be absent or "alternate" (in Atom these are semantically equivalent)