Element-wise absolute value operation!

I just added the ABS operation to Eltwise Ops. It's useful for implementation of Siamese Networks.

* It has both GPU and CPU implementations.
* I have provided the appropriate unit tests.

This isn't needed. `NeuronLayer::Setup()` takes care of it. https://github.com/BVLC/caffe/blob/master/src/caffe/layers/neuron_layer.cpp#L14-L17