Prevent error when using --api & comments have no example code.

Output if trying to use `--api` flag on anything that doesn't have example code in the comments:


```php
/usr/local/share/npm/lib/node_modules/dox/lib/api.js:25
  code.forEach(function(block){
       ^
TypeError: Cannot call method 'forEach' of null
    at Object.module.exports [as api] (/usr/local/share/npm/lib/node_modules/dox/lib/api.js:25:8)
    at Socket.<anonymous> (/usr/local/share/npm/lib/node_modules/dox/bin/dox:46:30)
    at Socket.EventEmitter.emit (events.js:93:17)
    at Pipe.onread (net.js:418:51)
```
[The offending line](https://github.com/visionmedia/dox/blob/26e3f936a9f290a755bbb2787a5f1cd250ca221b/lib/api.js#L25):
```js
var code = buf.match(/^( {4}[^\n]+\n*)+/gm); // this may return null

code.forEach(function(block){
  var code = block.replace(/^ {4}/gm, '');
  buf = buf.replace(block, '```js\n' + code.trimRight() + '\n```\n');
});
```

Simply needs to ensure `code` is an array before doing the `forEach`. Patch includes previously failing test.