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)

Let's experiment:

1. Remove these 2 lines.

1. Run `./openbazaar help`. What do you see?

1. Run `./openbazaar puppies`. What now?