use disco proxy in http client based on DISCO_PROXY

    DISCO_PROXY=http://localhost:8090 DISCO_PORT=8999 $GOPATH/bin/jobpack -W $GOPATH/src/github.com/discoproject/goworker/examples/count_words.go -I http://discoproject.org/media/text/chekhov.txt -C ~/foo

Correctly uses DISCO_PROXY environment variable to talk to my ssh forwarded disco master.

Sidenote: This project lacks documentation, and many things are used as global variables. I would like to refactor it to make it more object oriented and similar to other Go projects. Is that ok? My main concern is to make it more easy to submit Disco jobs from within other Go projects.