nan: auto-delete weak callback object

* Check with IsNearDeath() if it's safe to delete the weak callback
  context object.

* Turn _NanWeakCallbackData::Dispose() into a deprecated no-op,
  resource management is now handled automatically.

Incidentally fixes a double-free bug when the user calls Dispose()
twice.
Merge pull request #121 from bnoordhuis/persistent-isneardeath-auto-cleanup

nan: auto-delete weak callback object