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.
whoops, my bad, I think I was thinking of `caffe_gpu_powx`.  `caffe_gpu_exp` should probably exist but device abstraction (#610)  will probably take care of this so never mind, sorry!