Previously, if you didn't specify a list of `event_types` in either `register` or `call_on_each_event`, you'd get an exception due to the invalid event format.
This fixes#86.
In order to keep all three packages (zulip, zulip_bots,
zulip_botserver) in the same repo, all package files must now
be nested one level deeper.
For instance, python-zulip-api/zulip_bots/zulip_bots/bots/, instead
of python-zulip-api/zulip_bots/bots/.