0.5.2-wip: make NanAsyncWorker.*Persistent() public

[rvagg]

A small change to NanAsyncWorker, I'd like to make SavePersistent() and GetFromPersistent() public so I can use them from the outside. I'm finding that best practice might be to save a persistent of the parent object on each async call so GC doesn't get a chance to clean up the parent if there is no reference left in JS-land. For instance:

function () {
  var batch = db.batch()
  batch.put('foo', 'bar')
  batch.write(function () {
    // no JS reference here, `batch` is free to be cleaned up prior to this
    // if the `write()` call takes a significantly long time
  })
}

so instead of resorting to referencing hacks in JS-land, if you put the reference to the parent object, in this case batch into the NanAsyncWorker, in this case for the write() work, then it is kept around until the worker is cleaned up.

This is much cleaner if you can do this from outside NanAsyncWorker.

Having said all that, I think my projects might be the only ones actually using NanAsyncWorker so this is mainly FYI but I'd appreciate any feedback, comments or objections before merging.