Reduced memory consumption.

Removed the Feedparser.nodes property, which stored every node in memory. This property wasn't used anywhere in the code and wasn't documented.
Improved stream watermark and buffering.

- Feedparser is a Transform stream that has a writable stream (ie. its input) that takes bytes, and a readable stream (ie. its output) that is in objectMode: how to achieve this difference is documented here: http://nodejs.org/api/stream.html#stream_state_objects
- In objectMode the watermark counts the number of buffered objects (and not the number of bytes), we need to reduce it as not to store 16k of fat JSON objects in memory.