src: fix build error with nan 1.1.x

Move the weak callback out of the class again.  Fixes the following
build error:

    ../src/binding.cc:120:3: error: static and non-static member functions with the same parameter types cannot be overloaded
      NAN_WEAK_CALLBACK(WeakCallback)
      ^
    ../node_modules/nan/nan.h:1429:28: note: expanded from macro 'NAN_WEAK_CALLBACK'
        static NAN_INLINE void _Nan_Weak_Callback_ ## name(                        \
                               ^
    <scratch space>:112:1: note: expanded from macro '_Nan_Weak_Callback_'
    _Nan_Weak_Callback_WeakCallback
    ^
    ../src/binding.cc:120:3: note: previous declaration is here
      NAN_WEAK_CALLBACK(WeakCallback)
      ^
    ../node_modules/nan/nan.h:1417:28: note: expanded from macro 'NAN_WEAK_CALLBACK'
        static NAN_INLINE void _Nan_Weak_Callback_ ## name(                        \
                               ^
    <scratch space>:110:1: note: expanded from macro '_Nan_Weak_Callback_'
    _Nan_Weak_Callback_WeakCallback

Affects at least gcc 4.2 and 4.8 on OS X and Linux.

Fixes #92.