Implemented a feature to get partial data from a read stream

Implemented a feature to get partial data from a read stream.

Personally, I wrote this code to stream H.264 videos with Express web framework. To support H.264 video streaming in Google Chrome, I should have implemented HTTP `206 Partial Content` response. This code fits nicely with that.

I changed inner `mongodb` module from `1.1.11` to `1.4.0`. In previous version, `data` event is not called when using `seek()` function.