Add statusnet: prefix for API to JSON and XML output

Add statusnet: prefix for API to JSON and XML output

I added the statusnet: prefix to the xml output. This prefix should
be declared on the root element of all relevant XML output.

I also added two StatusNet-specific fields:

* statusnet:html - rendered HTML. Clients shouldn't have to guess
  at the correct HTML rendering for notices, especially since some
  of the links depend on context.
* statusnet:profile_url - profile URL for a user. You can't count
  on a user being a local user in a distributed microblogging world.
  So, this shows the explicit profile_url.