Added CommandSender.sendXmlMessage etc

Bukkit PR: [#1008] (https://github.com/Bukkit/Bukkit/issues/1008)
CraftBukkit PR: [#1320] (https://github.com/Bukkit/CraftBukkit/issues/1320)
Ticket: [BUKKIT-5245] (https://bukkit.atlassian.net/browse/BUKKIT-5245)

Please discuss on the ticket.