Allow merging events from different machines

[atenart]

Events manually collected on more than one machine could be merged to provide a unified view. For this to happen a few things are required:

• Provide a machine identifier in the events, so events can be distinguished. E.g. use the hostname by default?
• Support merging events at post-processing time, embedding the machine id in each event.
• UTC time should probably be used to allow sorting events in order.