pull() without a length does not pull until end of stream

https://github.com/EvanOxfeld/node-pullstream/blob/master/pullstream.js#L46 calls self.read() if len is not set. Node docs http://nodejs.org/api/stream.html#stream_readable_stream says, "If you do not specify a size argument, then it will return all the data in the internal buffer."

I ran into this with a big stream, it just returned 16,384 bytes.
