out = w * input
where w is a parameter of size 1

Change-Id: Ife682d62323ceb1a20cbbf6269421b20a862d888

* Check all files by pre commit hooks

Making '*' support the multiplication between a scalar and LayerOutput

Also changing '+' to support adding between a vector and a scalar.

Change-Id: I7daf35590dc2b2f855a29d9ef43ac57979442e0f

Update cluster_train.md for easier understanding

Currently, Paddle on Mac OS X is not deliberate testing through the different versions of  Mac OS X and Clang.
When all these things that we've done, we will reopen Mac build docs.

And rebase

Change-Id: I7ea234b3aa8fc70675af15d91db08242c43fb5ff

* Add elementwise math operations
This allows use to use expressions like: y=log(1+exp(x))
Also added unittests for ActivationFunction
* Enforce keyword arguments for non-positional arguments
* Add LogActivation to doc