Channel softmax

In this pull request, the behavior of SoftmaxLayer is changed from softmax over `channels*height*width` elements (all elements within a num) to softmax over `channels` elements (all elements at a spatial position within a num). This is for the purpose of running fully-connected layers as convolutions (see Net Surgery: http://nbviewer.ipython.org/github/BVLC/caffe/blob/master/examples/net_surgery.ipynb). It won't damage existing caffe examples, since fully-connected layer top blob has `width==1` and `height==1`.

The CPU version is implemented by @longjon and I implemented the GPU version, including GPU backward.
This kernel is the same as `caffe_gpu_exp` isn't it? Let's remove it and replace with caffe_gpu_exp, unless I'm misunderstanding somehow.  (I know it wasn't added this PR, but I just noticed it from seeing the diff.)