snapshot: adding snapshot.toJSON convenience method

[thlorenz]

In some cases you want JSON send or write to a file.
The intuitive way is to JSON.stringify a snapshot object but that fails due to circular references.

This method makes that easier.

[thlorenz]

BTW if you want I can also supply a snapshot.toStream(stream) convenience which could of course also be used to achieve the same result without holding the entire snapshot in memory.

[3y3]

Hello, @thlorenz , sorry for pending this pr. I have a lot of work on node-inspector.

I see a problems with using reserved method toJSON. What happens if someone will try to use JSON.stringify(snapshot)? At current time we'll receive an error about circular references, but after patching we'll silently pass this call.

I'd like to see also stream feature. How about a method that can work with callback and stream?
I'd like to have snapshot.export method and use it like:

var snapshot = profiler.takeSnapshot();
snapshot.export(function(err, result) {});
snapshot.export(writeStream);
snapshot.export().pipe(readStream||writeStream)

Something like this:

function ExportStream() {
  Stream.Transform.call(this);
  this._transform = function noTransform(chunk, encoding, done) {
    done(null, chunk);
  }
}
inherits(ExportStream, Stream.Transform);

/**
 * @param {Stream.Writable|function} dataReceiver
 * @returns {Stream|function}
 */
Snapshot.prototype.export = function(dataReceiver) {
  dataReceiver = dataReceiver || new ExportStream();

  var toStream = dataReceiver instanceof Stream,
      chunks = toStream ? null : [];

  function onChunk(chunk, len) {
    if (toStream) dataReceiver.write(chunk);
    else chunks.push(chunk);
  }

  function onDone() {
    if (toStream) dataReceiver.end();
    else dataReceiver(null, chunks.join(''));
  }

  this.serialize(onChunk, onDone);

  return dataReceiver;
};

In future this allows me to implement also snapshot.import;
If you agree, I'm ready to prepare new pr for your review.