Friendlier `--dev-mode` error message, new `help` command

![screen shot 2014-10-17 at 7 46 39 am](https://cloud.githubusercontent.com/assets/163977/4678263/574fab74-55f3-11e4-9630-4f6169e9d584.png)

It would make sense to show and error message when we run `./openbazaar puppies` with a couple of lines saying "type openbazaar help to see all available options despite the fact that ./openbazaar start is most probably what you need"