User data: getTokenOwner

[chrisabruce]

Gets information about the owner of an access token.

Gets information about the owner of a given access token.

Note that, as with the rest of the Client-Server API,
Application Services may masquerade as users within their
namespace by giving a user_id query parameter. In this
situation, the server should verify that the given user_id
is registered by the appservice, and return it in the response
body.