SIMPSONVILLE, S.C. — Gov. Rick Perry of Texas is taking significant steps to redefine his candidacy as his battle for the Republican nomination intensifies, including unveiling a tax overhaul plan that would give people the option of paying a 20 percent flat tax on income. .........Among the new faces who are joining Mr. Perry’s campaign is Joe M. Allbaugh, who managed the 2000 presidential race of George W. Bush and will be a senior adviser to the campaign. Mr. Allbaugh served as a director of the Federal Emergency Management Agency in the Bush administration and has been a longtime associate...