Nikkei rises to 5-1/2-year high as exporters and brokers surge

* Nikkei may reach 15,000 this week - broker * Exporters, brokers lead the gains * With USD/JPY assumption at 100, forecasts will rise 28 pct - analyst By Ayai Tomisawa TOKYO, May 13 (Reuters) - Japan's Nikkei share average surged to a fresh 5-1/2-year high on Monday as the weakening yen further bolstered exporters, while brokerage shares attracted buyers as growing volume was expected to bring in more fee income. Strength in Wall Street on Friday also spurred buying after