Build for CPU only

Now, the minerva can build on CPU only.

I added SigmoidForward, ReluForward, and TanhForward CPU basic implementation.
For apps, the minst_mlp works fine.

To-do:
* other CPU implementations including conv, backward...
Could we do something like in `unittest_elewise.cpp` to create separate tests, and disable GPU ones using the macro `HAS_CUDA`?