Add an Interval to Request and Reply payloads to limit responses to that interval if present

From the EITC Meeting discussion 20111027.

Add an xcal:IntervalType [0..1] to each Request payload, and each Reply payload where appropriate, to indicate that only results that touch that time interval should be included.

This allows (e.g.) EiRequestEvent to reply with only the events that are (say) current and in the next week, or alternatively, any past events that you know about.

This also addresses issues related to EiStatus (see ENERGYINTEROP-595) and the proposed EiPushPendingEvent operation (see ENERGYINTEROP-600 )