Add environment variable interface

Enable test_sgd_op

Fix sgd

fix EXTERNAL_LIBS in CMakeLists.txt

Rename `AsNoGradient` of VariableBuilder to `NotInGradient`

Remove flag use_mkldnn_wgt

Add MeanOp's Gradient Test And Fix Mean Op Gradient

Compare the gradient consistency between GPU and CPU calculations

Add memory.h for unique_ptr

Feature/remove shared ptr

FIX: Release CPU/GPU memory at the end of the Program 

Merge branch 'feature/remove_shared_ptr' of github.com:reyoung/Paddle into feature/remove_shared_ptr

Fit Google C++ Style

* add_op could take a unique_ptr or a const reference. If unique_ptr is
  taken, the NetOp will take care of that operator's life cycle. If a
  const reference is taken, that op will be Cloned.

Feature/clone op