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.

No need for this check. It's handled by `CheckBlobCounts()` https://github.com/BVLC/caffe/blob/master/include/caffe/layer.hpp#L34-L37