Add details for RNN tutorial

[titipata]

See post by http://www.wildml.com/2015/09/recurrent-neural-networks-tutorial-part-1-introduction-to-rnns/

So here are details that we should add:

• Add more RNN introduction
• Back propagation through time (BPTT)
• Loss function explanation
• Gated Recurrent Unit (GRU) and LSTM (http://arxiv.org/pdf/1406.1078v3.pdf)

[titipata]

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