rnn, lstm and scrnn are sharing a lot of duplicated code

[futurely]

wc -l rnn.lua => 365
wc -l lstm.lua => 294
wc -l scrnn.lua => 361
fgrep -xf rnn.lua lstm.lua | wc -l => 191
fgrep -xf rnn.lua scrnn.lua | wc -l => 228
fgrep -xf lstm.lua scrnn.lua | wc -l => 269