fix $0 on windows

$0 is crashed on windows!

for example, `d:\src\js\usage.js` is

```js
require('optimist')
  .usage('Usage: $0')
  .showHelp();
```

I run `cd d:\src\js && node usage.js` on windows and prints

```
Usage: node d:\src\js\usage.js
```

I want to print `Usage: node ./usage.js`, but printed is not.

so, I fix $0 to print what I want.

(note: I am Japanese high-school student, and my English is not so good. sorry)

my environment
  - windows 8.1
  - node.js v0.10.25
  - optimist 0.6.1