Adding learning rate decay
Adding learning rate decay


Former-commit-id: a55f31f98430c173fff848553594e40fa94518cd
danielhers/dynet,clab/cnn,chunyang-wen/dynet,clab/dynet,danielhers/dynet,chunyang-wen/dynet,chunyang-wen/dynet,danielhers/dynet,danielhers/dynet,clab/dynet,clab/cnn,shuheik/dynet,clab/dynet,clab/cnn,danielhers/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,shuheik/dynet,clab/dynet,xunzhang/dynet,clab/dynet,xunzhang/dynet,shuheik/dynet,chunyang-wen/dynet,xunzhang/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,xunzhang/dynet