Nan upgrade

- [ ] Fix the failing test between v0.10 and v0.11

@TooTallNate any idea why this test https://github.com/deepak1556/node-ffi/blob/nan_upgrade/test/foreign_function.js#L34 passes on v0.10 but not on v0.11. As it turns out v0.10 errors here https://github.com/deepak1556/node-ffi/blob/nan_upgrade/lib/_foreign_function.js#L51 but not on v0.11 where the `result.deref()` returns `0` . Why is this inconsistency ? Is it because of using `int64_t` https://github.com/TooTallNate/ref/blob/6d4b9362ecba1e3c650c86502ba8ca6e8847da49/src/binding.cc#L297 here ? Would be nice to fix those test and support v0.11 :)