It is not guaranteed that the integration scripts in
the Zulip repository even specify a `provision` option.
Therefore, checking the value of this option would fail.
Updating this with getattr and a default value.
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/.