Make CreateOp in Plain C++ params

Read/Write a Tensor Python

Change `in_out_idxs_` to shared_ptr

Add gometalinter errcheck to pre-commit-hook, fix all Go unchecked errors.

* `in_out_idxs_` shares between all operator instance in same type
  of operator.

Fix Merge Bugs

Net::DebugString() print what it is and invokes inner operators
DebugString() with a prefix '\t'.

* There is a merge conflict when merge PR #2914
* Develop and PR #2914 both add `DDim::size` method, but did not
  triger git merge conflict while merge.

Add skeletons of `mul`, `rowwise_add`, `sigmoid`, `softmax` ops

* Follow review comments to seperate Tensor Numpy interactive methods in
  tensor.h.
* Simplify logic for `CastToPyBufferImpl`, make it as one struct and in
  details namespace.
* Remove `Scope` expose in Python, since it currently is useless.
* Remove some debug functions.

Fix some link errors about NNPACK.

refine tensor's conditional compilation, remove Tensor::numel_ and add DDim::size()

Unchecked errors could be handled by: cd go; gometalinter --vendor --disable-all --enable errcheck $(glide nv)

Basically following
http://pybind11.readthedocs.io/en/stable/advanced/pycpp/numpy.html

* Use buffer protocol to return a view of Tensor. It can be cast to
  numpy array in Python.
* Set a numpy array to a tensor.

Enable tensor test

* Also fix unit test

Python Generate OpCreation Methods by OpProto

* add inputs

* add ut for multiple inputs

* fix AddToLayer

* op_desc -> op_proto

* CreateArgumentOffsetMap -> CreateInOutOffsetMap

* move CreateInOutOffsetMap from OperatorBase to op registry

* arg_idxs_ -> in_out_idxs_

* Implement InferShape and register them, give a stub Kernel method
  by LOG(INFO)

CMake `op_library` function

Update the dockerfile to build cross-compiling image for Android.

Refactor DDim's product() and add slice_ddim()