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
Forgot to answer the initial question. m)

> Why would this cast be needed? Any pointer is a void pointer.

In terms of overloads you are right. Any pointer is implicitly converted to `void*`. But while matching templates `int*` and `void*` are two completely unrelated things.