Fix windows link error by expose the entry point for REGISTER_NODE_TYPE

[souptc]

This PR is to fix issue #1547 , The original plan is to expose the dmlc::Registry::Get symbol in dmlc-core project, but later I found it does not work because nnvm is not only the consumer of dmlc::Registry symbol, but also need to enable registry on some new types, like:
DMLC_REGISTRY_ENABLE(nnvm::PassFunctionReg);

If we add MACRO like DMLC_DLL in registry.h, it will fail in nnvm because Windows does not allow to implement a function that declared as dllimport. I am currently use a workaround that in tvm lib, we expose another method that allow other component to access the dmlc::Registry<::tvm::NodeFactoryReg> instance, so nnvm could consume that one instead directly invoke the Get method in dmlc header file.

This is the simplest solution I can think of at this moment, but not sure does it align with tvm's design. Let me know if you have any concern.

[tqchen]

Thanks @souptc for the contribution! This works, and is merged