Port Fully Connected operator

[Xreki]

Port Fully Connected operator to the new framework. The computation must be totally the same as the old FullyConnectedLayer, that is Out = sum_n{X_i * W_i} + b.

• Support different kinds of activation functions, such as identity, sigmoid, softmax.
• Implemented by calling other operators (MulOp, RowwiseAddOp, activation ops).
• Support multiple inputs and weights.

Problems I met during porting FCOp are recorded in several issues:

• OpRegistry::CreateOp调用，输入/输出名字写错，报错信息难以理解 #3899
• 是否需要统一Op输出的名字？ #3922