Add event IDs to CF Stack events (#852)

So that events can be uniquely identified.

I tried to match the format documented here:
http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/using-cfn-listing-event-history.html

tests/test_cloudformation/test_cloudformation_stack_crud.py

@@ -522,6 +522,7 @@ def test_describe_stack_events_shows_create_update_and_delete():
         for event in events:
             event.stack_id.should.equal(stack_id)
             event.stack_name.should.equal("test_stack")
+            event.event_id.should.match(r"[0-9a-f]{8}-([0-9a-f]{4}-){3}[0-9a-f]{12}")
 
             if event.resource_type == "AWS::CloudFormation::Stack":
                 event.logical_resource_id.should.equal("test_stack")