DepthConcat

This adds the DepthConcat module which is used in the Inception layers used in the [Going deeper with convolutions](http://arxiv.org/pdf/1409.4842v1.pdf) article. With this Module, we should be able to build the Inception/GoogleLeNet model using torch/nn. 

The code is unit tested and documented. It is general enough to be used for temporal, spatial, volumetric, and other such convolution-like concatenations.