Shares of staffing firms Robert Half International (RHI), ManpowerGroup (MAN) -- the largest U.S employment services company -- and others surged to near-term highs after the government reported that employers added 155,000 jobs in December. Staffing firms have grown rapidly in recent months amid signs that the job market is slowly improving. The Labor Dept. report Friday morning confirmed the trend, and said growth was paced by improvement in the