Replace `OperatorPtr` with `std::shared_ptr<OperatorBase>`

Remove ScopePtr and OperatorPtr

* ScopePtr means pointer of scope, but it can be shared or uniqued.
Change it to std::shared_ptr<Scope> to make code better to read.

* Expose Net to Python

* Expose PlainNet to Python, make python can add_op, complete_add_op
* Provide a low level api to manipulate Net
* Unittest for Net::DebugString

Fix memory deps on device context

Move Copy out from memory.h into memcpy.h

Slight simplification of eigen.h in hope to make it more readable.

Add CPU/GPU Memcpy in memory folder

Simplify Tensor implementation

enable Intel MKLDNN library and Intel MKL small package

Mobilenet gpu implementation

Implement some basic op kernel

1. Change PODDeleter's template parameter 'PlaceType' to 'Place'.

2. Limit PODDeleter and Tensor::mutable_data()'s `T` to POD type.

* add cross-entropy-op

* add infershape and compute

* implement Infershape and compute of onehotcrossentropy op

fix client discover pserver context cancelled

It's already fixed by Wuyi's PR, but his PR may take some time to
merge, but I want to get this change in ASAP.