Add benchmarks for RNNs (#80)
Add benchmarks for RNNs (#80)

* Add benchmarks for RNNs

* Convering the three most frequently used RNN cell types:
  * SimpleRNN
  * GRU
  * LSTM
* Now the benchmarks "demo" is complete in terms of covering all
  three major types of common models: MLP, convolutional and RNN.


tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs