Added DateTime to events.

Added a DateTime field member to events to add the option to create events with a different start and stop times oher than DateTime.Now.

When implementing the allure-mstest-adapter and the allure-vstest-logger I had to derive from several events adding the DateTime field so I figured that might be useful for other projects too.
Either way if its used by two project I think it should be placed in allure-csharp-commons.