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.

There is already an absolute value function in Caffe: `caffe_{cpu,gpu}_fabs()`. However, I like `abs` alone better as a name and prefer your MKL / non-MKL implementation to `fabs` everywhere. Please drop `caffe_{cpu,gpu}_fabs()` and replace instances in the code with your `caffe_{cpu,gpu}_abs()`.