fixed crash when calling SDL.IMG.load

In node v0.10.29 I was doing something like:

    var surface = SDL.IMG.load("./images/logo.png");

which always led to this crash:

    node: /usr/include/nodejs/src/node_object_wrap.h:71: void node::ObjectWrap::Wrap(v8::Handle<v8::Object>): Assertion `handle->InternalFieldCount() > 0' failed.

After modifying sdl::IMG::Load() to use the NEW_WRAPPED macro, I can successfully use images.