New eventCreatedOn attrib for GerritTriggeredEvent

In certain situations, it has become necessary to know
when an event was created on the Gerrit Server side.

This attribute will be available in Gerrit 2.11
I use it in the Gerrit Trigger unit tests. 

Essentially, the attribute should only be set via the event factory as it receives it from json. Never via the setter...except for a unit test.

I can update the text as you see fit.