Add changelog for atexit fix to threading handler.
Add changelog for atexit fix to threading handler.

harlowja/kazoo,AlexanderplUs/kazoo,bsanders/kazoo,Asana/kazoo,max0d41/kazoo,rackerlabs/kazoo,kormat/kazoo,pombredanne/kazoo,jacksontj/kazoo,harlowja/kazoo,python-zk/kazoo,rockerbox/kazoo,kormat/kazoo,rockerbox/kazoo,tempbottle/kazoo,rackerlabs/kazoo,rgs1/kazoo,tempbottle/kazoo,jacksontj/kazoo,pombredanne/kazoo,max0d41/kazoo,AlexanderplUs/kazoo,rgs1/kazoo,python-zk/kazoo,bsanders/kazoo