recur-nn: add clipped ReLU activation

This is a ramp function:

  x <= 0         ->   0
  0 < x <= 20    ->   x
  20 < x         ->  20

as used for RNN speech recognition by Maas, Hannun, Jurafsky, Ng in
http://arxiv.org/abs/1408.2873.  At least I think this is what they
mean.

It is similar to the piece-wise clipping that Mikolov used way back.