use version information from package.json file

There are two version strings in pm2 right now: version of npm package is 0.4.3, and "pm -V" prints out 1.0.3.

I suppose it's unintentional, so I suggest to use version from package.json everywhere.

So we can require package.json and get version information from there. I got this idea from "express" source code:
https://github.com/visionmedia/express/blob/master/bin/express#L10

Too bad that commander module can't just do it automatically. :(