update download script to resume downloads when server supports it (fix #559)
update download script to resume downloads when server supports it (fix #559)

ros/catkin,wkentaro/catkin,asmodehn/catkin,scpeters/catkin,harmishhk/catkin,asmodehn/catkin,scpeters/catkin,thaddeustaylor/catkin,ros/catkin,shyamalschandra/catkin,shyamalschandra/catkin,beni55/catkin,thaddeustaylor/catkin,beni55/catkin,thaddeustaylor/catkin,wkentaro/catkin,shyamalschandra/catkin,asmodehn/catkin,scpeters/catkin,jack-oquin/catkin,harmishhk/catkin,beni55/catkin,jack-oquin/catkin,ros/catkin,harmishhk/catkin,jack-oquin/catkin,wkentaro/catkin