Add support for events GUILD_BAN_ADD and GUILD_BAN_REMOVE about discordgo

[bwmarrin]

{"t":"GUILD_BAN_ADD","s":133,"op":0,"d":{"user":{"username":"SomeCoolBot","id":"123487981234723","discriminator":"1234","avatar":null},"guild_id":"81273912873"}}

[bwmarrin]

{"t":"GUILD_BAN_REMOVE","s":177,"op":0,"d":{"user":{"username":"SomeCoolBot","id":"123487981234723","discriminator":"8292","avatar":null},"guild_id":"81273912873"}}

[iopred]

I was going to take this, but it looks like it's done.

[bwmarrin]

It's not done :) These are the websocket events that get fired when someone is banned/unbanned. The code that does the actual ban/unban is in restapi.go but this is for an addition to the wsapi.go file in the event func. So just a new similar block of code as the rest and then adding a user callback func variable to the structs.go Session struct. Looks like they can share a struct so just make one that works for both should do the job when sending the data to the (if defined) user callback func.

Also, btw - Thanks a lot for all the help. I owe you a few beers at this point.