Squashing everything into a single commit!
Added Eltwise absolute value operation, usefull for implementation of siamese networks\!

Added the MKL alternative for Abs

Removed the old implementation of fabs to replace it with MKL/non-MKL/GPU abs operation

Added Eltwise absolute value operation, usefull for implementation of siamese networks\!

Added the AbsVal Layer, replaced the default fabs

Fixed the the style

Revert the changes in test_eltwise and vision_layer