Add unpooling operator for segmentation network

[futurely]

#422 and #503 have added the deconvolution operator for segmentation. The other missing piece to construct a complete segmentation network is the unpooling layer (#337 (comment)).
Current reference implementation is only in Caffe.
https://github.com/HyeonwooNoh/caffe/blob/new_data_layer/src/caffe/layers/unpooling_layer.cpp
https://github.com/HyeonwooNoh/caffe/blob/new_data_layer/src/caffe/layers/unpooling_layer.cu

[tornadomeet]

yes, when using the Deconvnet for segmentation, the last op is uppooling, but currently, the io part for segmentation is not support.
if the mxneters agreed adding the uppolling op, i will try it.

[tqchen]

Yes, that would be great

[tornadomeet]

i just see this, i'll do it this week~

[antinucleon]

Done. #746