Layers in order

In reponse to @shelhamer's [advice](https://github.com/BVLC/caffe/pull/522#issuecomment-47252852), ArgmaxLayer, ConcatLayer, FlattenLayer, SoftmaxLayer and SplitLayer are moved into common_layers. ConvolutionLayer, EltwiseLayer, Im2colLayer, InnerProductLayer, LRNLayer and PoolingLayer stay in the vision_layers. But your mileage may vary.

The data layers and the loss layers are in alphabetical order with the exception that the LossLayer is above its subclasses.