fix segfault in GlobalPropertyGetter() (closes #89)

We operated on `wrapper` (the result of `createDataWrapper()`),
not JavaScript instance of `ContextifyContext`, when we did
`ObjectWrap::Unwrap<ContextifyContext>(data)` in `GlobalPropertyGetter()`.
As a result, we were receiving pointer to a destructed instance after
`ContextifyContext` instance was disposed of.