Provide start/end date time via Conduit for Calendar

Summary: Fixes T11706. I think this approach (roughly: provide the information in a few different formats) is generally reasonable, and should let clients choose how much date/time magic they want to do.

Test Plan: Called calenadar.event.search, viewed results.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11706

Differential Revision: https://secure.phabricator.com/D16688