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.
Updated readable side highWaterMark to be forward-compatible with node.

Since https://github.com/joyent/node/commit/ba72570eae938957d10494be28eac28ed75d256f, node's Stream will have a default watermark of 16 if in objectMode.
Revert "Reduced memory consumption."

This reverts commit bd74a78f0ec0b1707e74424f26f010614ee89302.
Merge remote-tracking branch 'PaulMougel/master'
Revert "Revert "Reduced memory consumption.""

This reverts commit 89066e7f5913d36912bdf7f50636e37a2f27ebd0.