The merger finalized Tuesday between Boston’s NStar and Connecticut’s Northeast Utilities has created one of the nation’s largest energy delivery companies, with more money to invest in system improvements, more crews to fix power outages, and better bargaining power to negotiate lower rates for customers, chief executive Thomas J. May said. NStar is now a subsidiary of the combined utility, which will be the region’s largest, serving nearly 3.5 million electric and gas customers in Massachusetts, Connecticut, and New Hampshire.