Added 4 new events involved in XRDS: StartUserXRDS, EndUserXRDS, StartPublicXRDS, EndPublicXRDS
Added OpenID provider functionality (no delegation support [yet])
As a first step to pluginizing our OpenID support, I've moved the
important OpenID-related files to a dedicated plugin directory. Many
of these classes are still referred to by libraries that are still in
core.