make feedparser a transform stream

You are so very close to have a real transform stream, this takes the last step and uses the node 0.10, transform class.

I have tried hard to maintain backwards compatibility, only if the user listens to `end` there can be a difference. But I really think you should drop all those callbacks and extra methods (`parseUrl`, `parseString`, `parseFile`) and make this a pure transform stream. Users can then use stream/buffer tools like `createReadStream`, `request`, `endpoint` and `startpoint` to get the similar results.

_Note that your tests on master (and this branch too) seams to be unstable!_