Update for node v0.8.20

### Changes since v0.8.18
* npm: Upgrade to v1.2.11
* http: Do not let Agent hand out destroyed sockets (isaacs)
* http: Raise hangup error on destroyed socket write (isaacs)
* http: protect against response splitting attacks (Bert Belder)
* zlib: pass object size hint to V8 (Ben Noordhuis)
* zlib: reduce memory consumption, release early (Ben Noordhuis)
* buffer: slow buffer copy compatibility fix (Trevor Norris)
* zlib: don't assert on malformed dictionary (Ben Noordhuis)
* zlib: don't assert on missing dictionary (Ben Noordhuis)

(from http://blog.nodejs.org/)