Support gzip with encoding on node pre-v0.9.4

setEncoding was added to zlib streams in joyent/node@9b5abe5 (v0.9.4 and
later).  In order to support encoding and gzip together on earlier
releases, pass the data through an additional Stream which decodes the
data using the stringstream module.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>