GRU support

[kylebgorman]

It would be nice if our RNN encoders and decoders, which are currently LSTMs, could be replaced with GRUs.

A simple CLI option would be something like this. Rename the architectures Rnn and AttentiveRnn and modify the architecture options to reflect the change. By default, make the RNN type default to LSTM, but add a flag (--rnn_type? just --rnn) to select a GRU instead.

Alternatively, but only if necessary, one could project this choice down into the architecture level so that one can choose between an AttentiveLstm and an AttentiveGru etc.

I am assigning this to @bonham79 since I know it's an interest of his.