Enclosure support

I found my self in need of having enclosures in my RSS-feeds. I found the forked version of this module made by djthread but I wasn't really satisfied with the way that the enclosure support was implemented. Here's my implementation.

```js 
// Use a file as enclosure and the module will figure out 
// the mime type and file size
feed.item({
  title : 'The title',
  ...
  enclosure : {url: 'url-to-file', file : 'path-to-file'}
});

// You can also declare all the attributes yourself 
feed.item({
  title : 'The title',
  ...
  enclosure : {url: 'url-to-file', type:'image/png', length:93133}
});
```

*Unit tests is added and all is green :)*