to be scheduled at a later time. The interface to event_schedule is
identical to event_notify, except it takes an additional struct
timeval argument that specifies when the event should be fired.  We
assume time synchronization between nodes.

* Revamped the attribute interface. Rather than a single get and put
function that takes a union "type" argument, we now have separate
event_notification_get_<type> and event_notification_put_<type>
functions, where <type> is one of "double", "int32", "int64",
"opaque", "string". These changes should greatly simply the attribute
interface. The opaque attribute type is new, and allows
arbitrary data structures to be added to notifications as attributes.

* Added event_notification_remove, which deletes an attribute from a
notification.

* Modified the event notification callback to take "host" and "type"
parameters, which contain the "host" and "type" attributes from the
event notification, respectively.