Removed greedy template definition of NanNew(...)

In #208 I proposed to remove the generic version of `template<typename T, typename P> v8::Local<T> NanNew(P arg1)` because it is error-prone.

This PR removes the function definition leaving only a declaration. It also adds the specializations required by the test suite.

Not sure I got everything right and to your liking, but I think it is enough to outline what needs doing.

HTH
But that is not a specialization, but an overload that is also a template... I think... I've been battling this problem in this very function over many iterations and still don't remember what is doable and what is not.