Add __declspec(dllexport) to class Node and class Op if build on Wind… (#296)

* Add __declspec(dllexport) to class Node and class Op if build on Windows, so that it can be referenced in some test code in MXNet

* correct typo

* reuse the macro in c_api.h

* revert changes on base.h

* one more place to revert

* use enclosing extern c block

* revert a change caused by copy

* remove a space