Add changelog for Zulip 1.3.13 release.

Add changelog for Zulip 1.3.12 release.

Add changelog for Zulip 1.3.11 release.

Add changelog for Zulip 1.3.10 release.

Add changelog for Zulip 1.3.9 release.

Add changelog for Zulip 1.3.8 release.

Rewrite the email gateway integration instructions.

Fix validation that twitter creds are present.

They are looked up as secrets which initialize to `None`, but the code
was checking for empty strings.

This, along with zulip#80, fixes zulip#81.

puppet: Move /var/lib/nagios_state creation to zulip::base.pp.

Previously, in Zulip voyager, the cron jobs would spew error emails
every time they ran, due to this directory not existing.

This also tightens the permissions for the folder and avoids needing
to create a nagios user for Zulip voyager; it should be writeable by
both root and the zulip user and world-readable (and thus readable by
the Nagios user on zulip.com systems).

Add description of Zulip at top of README.md.

Based on PR zulip#22 by GedLawrenson.