* Do not use `virtualenv` in Docker
* Split unittest into many target, make them parallel

Add @EMPTY@ to default no_grad_vars in `Backward`

Fix CPU compile

error fix on the front page

update MKLDNN cmakes

Move paddle/framework/detail/tensor-inl.h into framework/tensor_impl.h

Add Gradient Operator for mean

Refine InferShape for recurrent_network_op

"add no gradient"

Add the check of cudnn version in cudnnBatchNorm

ignore im2col if not necessary in conv 1 * 1

Refine compute code in operators

Install GCC 4.8 in Dockerfile so to work with Eigen GPU code

Add ClipLayer

* export Backward to python

add RowL2NormLayer

Return Reference Instead Pointer to GetEigenDevice

add wget in docker production image

enable operator gpu unittest