GLOBAL MARKETS-Franc drops as Swiss announce negative rates; stocks jump after Fed

* Swiss franc tumbles as SNB introduces negative depo rate* Shares jump as Fed pledges to be patient on withdrawing stimulus* Wall St expected to rally again after biggest gain of year* Treasury yields edge up, oil up for second day* Rouble drops as Putin talks tough on rift with WestBy Marc JonesLONDON, Dec 18 (Reuters) - The Swiss franc tumbled on Thursday after its central bank announced a char