Simple LSTM Cell interface

I am adding a simple `LSTMCell` to dynet. It is a stateless LSTM unit that takes as input the current input `x_t` and previous state `(c_tm1, h_tm1)` and outputs `(c_t, h_t)`. This is similar to `nn.LSTMCell` in pytorch, and is useful in cases like beam search, where you need to dynamically keep the set of previous hidden states.

Details can be found [here](https://github.com/pcyin/dynet/compare/master...rnn_cell). I can do `RNNCell` and `GRUCell` as well is this is something useful to include in the official repo.