Created by: reyoung

Even Add is shorter than Append, Append is better than Add in several points:

1. Append hint the NetOp is a sequence of operators and append just add that operator at the end of NetOp. PushBack is a more C++ style name but Append seems simpler.
2. Net.AddOp is easy to confusing with element-wise add operator, which means add two or more tensor with the same shape together.

If that name should be changed, I will give a PR next day as my fix-it day PR :-)