More taxicabs are on the way to San Francisco after a recent study found a shortage of cabbies in the city.  In a plan approved yesterday, the SFMTA will put 120 taxis into service this year and 200 next year.  More cabs will be added annually to raise the current fleet of 1600 to 2300.  Some taxi drivers are concerned the influx and competition from ride-sharing groups could result in a smaller take-home pay.