add: A useful shell for unix-like system to control the client.

It will start the goagent silently on the background, and user can close the shell session without pain.
It should work on unix-like system, such as OSX, Archlinux, Ubuntu, etc.
**This shell has no dependency**.

Usage:

    ./proxy.sh
    ./proxy.sh stop
    ./proxy.sh restart

The default argument is `start`.