* Charity says 16 staff killed since 1991
* Islamist militants loot hospital, force patients out
* War-torn Somalia still faces stubborn insurgency
(Adds al Shabaab looting, quotes from district chief)
By Drazen Jorgic
NAIROBI, Aug 14 (Reuters) - A medical charity that was a lifeline for hundreds of thousands of Somalis announced on Wednesday it was pulling out of the country, ...