Add Event methods to db api.

Start of adding the ability to store raw events in Ceilometer.
Query Filter mechanism is crude for now. Need to find a more expressive
grammar than this EventFilter thing.

Change-Id: I3c64e3858df756d93dd5bf3c6c9651be25cd2b55
Blueprint: https://blueprints.launchpad.net/ceilometer/+spec/add-event-table