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)

so no help shown when on wrong command? just the:

`openbazaar.py: error: argument command: invalid choice: 'laskdjsakl' (choose from 'start', 'stop', 'help')`

error?