Add ElmanCell and ElmanRNN
Add ElmanCell and ElmanRNN

Summary: Closes https://github.com/caffe2/caffe2/pull/1742

Reviewed By: dzhulgakov

Differential Revision: D6706809

Pulled By: anderspapitto

fbshipit-source-id: 15a05786a26aeb719ea4377f4dbbb62738d9e697

Yangqing/caffe2,Yangqing/caffe2,caffe2/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2