* More than 100,000 have sought refuge at airport
* Heavy weapons fire rings out in Bangui for second day
* Children's clinic runs out of blood - aid group
By Paul-Marin Ngoupana and Serge Leger Kokpakpa
BANGUI, Dec 31 (Reuters) - Thousands of people caught up in fighting in Central African Republic blocked the runway of its international airport on Tuesday, demanding more aid and the ...