add support for @param tags w/o a type

According to [the JsDoc docs](http://usejsdoc.org/tags-param.html), the `@param` tag does not require a type. So this should be legal:

```javascript
/**
 * @param somebody
 */
function sayHello(somebody) {
    alert('Hello ' + somebody);
}
```

This patch updates doctrine to parse the `@param` tag just fine in this case, w/ a `type` of `null`.