add keep_events_for as an optional field for agents to decide how long to store events
fix spec?
typo
add validation and updating of existing event's expires_at values
Merge remote-tracking branch 'origin/master' into add_keep_events_for_to_agents

Conflicts:
	CHANGES.md
	app/models/agent.rb
fix specs
remove duplicate method
Merge branch 'master' into add_keep_events_for_to_agents
update Agent#events_count when events are expired
Merge pull request #118 from cantino/add_keep_events_for_to_agents

Allow agents to determine how long to keep events for