Clean up node-debug help message

- Reword so lines are closer to 80 chars wide (yargs does not wrap
  option descriptions).
- Use basename of $0 as command name, so full path
  (/usr/local/bin/node-debug) isn't printed in the usage.

Note that this syntax does not work:

```
node-debug --debug-brk script.js
```

This is basically a design error in optimist. The author assumes its possible to parse argv with no configuration, this is wrong. You need to know difference between flags (no argument) and options (have argument). In above, script.js is assumed to be argument to --debug-brk, so node-debug just does a require(.) and starts a repl.

In one case, luckily, optimist knows a flag is a flag: --no-debug-brk is treated as identical to --debug-brk=false, so the typical usage (now that debug brk is default), DOES work.