Added KindsOf method to Value type

[afinch7]

Pretty simple stuff just added a KindsOf method for the Value type, and a KindList type with a method String.

Should allow for a little better error reporting from bound functions.

I.E.

func someBoundMethod(in v8.CallbackArgs) (*v8.Value, error) {
    if !in.Arg(0).IsKind(v8.KindString) {
	return nil, errors.New("Expected String type for arg 0 but got: " + in.Arg(0).KindsOf().String() + ".")
    }
} 

I would imagine there are other uses this is just the one that motivated me to make this pr.