[rotang] Adding in the email job handler.

The first job checking.. Jobs are handlers that runs periodically.
This handler goes through all rotations sending notification emails to
upcoming on-callers.

This first CL does not consider the calendar entries with the accept/decline
state.


Bug: 845549
Change-Id: I74feea33a573152140692293f20f21d753b259e6
Reviewed-on: https://chromium-review.googlesource.com/1226728
Commit-Queue: Ola Karlsson <olakar@chromium.org>
Reviewed-by: Tiffany Zhang <zhangtiff@chromium.org>
Cr-Commit-Position: refs/heads/master@{#17648}