event: riemann_event_set_one() improvement

Instead of taking only one arg, take a variable number of them, so that
we can use this function to set tags and attributes too.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
src/riemann-send-events: Set a custom tag & attributes

Until user-settable tags and attributes are implemented, send a custom,
hard-coded list of tags and attributes.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>