added is_frozen() to Dict
added is_frozen() to Dict

cherryc/dynet,mitchellstern/dynet,dhgarrette/cnn,dhgarrette/cnn,dhgarrette/cnn,Peilun-Li/dynet,Abasyoni/dynet,yoavg/cnn,cherryc/dynet,Peilun-Li/dynet,yoavg/cnn,kamigaito/cnn,kamigaito/cnn,Peilun-Li/dynet,cherryc/dynet,Abasyoni/dynet,mitchellstern/dynet,Abasyoni/dynet