Added getKickedFrom() to ServerKickEvent.

Currently there's no clear way to get from which server user was kicked in ServerKickEvent.
This issue led to some creations like:

https://github.com/CraftMinecraft/MoveMeNow/blob/master/src/main/java/net/craftminecraft/bungee/movemenow/PlayerListener.java#L26-L38

that are not accurate (when State == Connecting).