Add ability to merge two objects.

@orenmazor @boourns You guys might need this functionality in the future if you're tracking anonymous and signed in users and need to merge timelines once they sign up/sign in.

This change also cleans up some code around inserting non-appended events into a timeline.

Oh, and I added a `Servlet.PutEvents()` which allows you to insert multiple events at a single time for a single object if you guys need that for bulk loading. That's not publicly exposed through the HTTP API but it could be.