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)

I agree to replace the assertion with `sys.exit(1)`. However, the message is over-detailed. Think about it: This error can only happen if a user has *manually* set `--disable-stun-check`. So, we can assume that:

1. they have already read the "usage" message, or they wouldn't have known this exists.

1. they did this intentionally.

So, the message of the assertion is all they need.