add bitcoin to the currency list
add bitcoin to the currency list

bitcoin is referenced using its financial symbol XBT
description is with a lowercase b because its about
the unit, not the trading system

libin/kanboard,BlueTeck/kanboard,BlueTeck/kanboard,kanboard/kanboard,renothing/kanboard,renothing/kanboard,kanboard/kanboard,libin/kanboard,renothing/kanboard,kanboard/kanboard,BlueTeck/kanboard