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)

Here:
```python
elif not arguments.dev_mode:
    if network_util.is_private_ip_address(arguments.server_ip):
        print "openbazaar: Could not start. The given/default server IP address",
        print  arguments.server_ip, "is not a public ip address."
        print "(Try './openbazaar help' and read about the '--server-ip', '-i' options)"
        sys.exit(1)
```