Backward

Fix doc

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.