Support apps.events.authorizations.list

[jsmestad]

Background

My attempt at adding support for apps.events.authorizations.list mentioned in #241

I had to update ex_doc to avoid a failure with mix docs. I also fixed a warning about a duplicate @doc string for the same method.

[jsmestad]

Unfortunately upon testing this, it looks like the Slack API endpoint only supports application/json in order to work. So this cannot get merged in till either Slack supports form encoded or this library supports json payloads.