Add support for notification events to the placement service

Should be a fairly self-contained piece of functionality... we want to add support for (versioned) notification events to the placement service.

Things needed:

* Short nova-spec submitted for Pike
* Drop/lift SOME of the code from nova/notifications/base.py
** Basically, we want the generic stuff, not the crufty old stuff :)
* Create versioned objects for various notification payloads (with with gibi -- Balasz Gibizer) on this
** placement.resource_provider.create
** placement.resource_provider.delete
** placement.resource_provider.aggregate.associate
** placement.resource_provider.aggregate.disassociate
** placement.resource_provider.inventory.update
*** NOTE: this event payload should show a BEFORE and AFTER view of the inventory
** placement.allocation.create
** placement.allocation.delete