Porting LSTM Operator

[qingqing01]

The lstm_unit_op calculates one time step for the input sequence in LSTM network. Different from it, this LSTM operator will calculate all time step for the input sequence.

• support variable-length base on LoDTensor.
• support peephole connection by default, the users also can disable it.
• support to set cell initialized state and hidden initialized state by users.
  • The LstmLayer in PaddlePaddle now set them to zeros by default：https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/gserver/layers/LstmLayer.cpp#L174
• support to pass the states across mini-batch.
• output cell state and hidden state.