Update activity streams JSON to match spec

Update activity streams JSON to match spec

Squashed commit of the following:

commit 0722450267a1d0f4bdc2853f52a85b850329db73
Author: Zach Copley <zach@status.net>
Date:   Thu Aug 25 09:58:29 2011 -0700

    Updgrade activity object json

commit 882ba1dceaba8a0b3ec3513760aa09f68e41f270
Author: Zach Copley <zach@status.net>
Date:   Wed Aug 24 16:30:07 2011 -0700

    Update to the JSON activity serialization document

commit 121e441b314b93e184711c3dcc79ada69d429eba
Author: Zach Copley <zach@status.net>
Date:   Wed Aug 24 15:08:06 2011 -0700

    Output application/json instead of application/stream+json (at least for now)

commit e045e214bffe5e0ddeb0a42555d440b75ae4edde
Author: Zach Copley <zach@status.net>
Date:   Wed Aug 24 15:06:40 2011 -0700

    Update to use latest property names from the JSON activity spec