pass wrapped function to onAsync function

Just and idea I had.

Pass the function being wrapped to your `onAsync` function. This is mostly useful for debugging purposes.

```javascript
function onAsync(f) {
  process._rawDebug('async function %s scheduled', f.name);
  return f.name;
}

var handleTrace = {
  before: function before(context, trace) {
    process._rawDebug('before async function %s invoked', trace);
  },
  after: function after(context, trace, returnVal) {
    process._rawDebug('after async function %s invoked', trace);
  },
  error: function error(trace, err) {
    process._rawDebug('error in async function %s invoked', trace);
  }
};

process.addAsyncListener(onAsync, handleTrace);
```