This commit adds support for the `afk_channel_id`, `afk_timeout`, and
`system_channel_id` parameters in the
[create guild](https://discord.com/developers/docs/resources/guild#create-guild-json-params)
endpoint by adding the `afkChannelID`, `afkTimeout`, and
`systemChannelID` options in `GuildManager#create`.
This commit also fixes a minor bug in `create` when specifying types for
the channels due to the channel types not being changed from `'text'`,
`'voice'` etc to the corresponding numbers, so Discord would return an
error.
