Fix Client#createGuild() bug where shard was undefined

[apacheli]

This fixes 2 issues that were recently discovered in the Eris channel.

1. The first issue was that it was a race condition between Guild#shard and Client#guildCreate.
2. The second issue comes from the first one. The guild obtained from REST was not cached.

[Skillz4Killz]

Why not have a property dedicated to the client? this.client = client;. You'd never have bugs like this again and users don't need to go guild.shard.client. One way or the other, client exists in the class.

Note: This needs typings updated to reflect the changes in the PR.

[apacheli]

Sorry for delaying so long. I completely forgot about this. Happy late holidays lol