Add possibility to specify which fields should be loaded

Before this commit it was impossible to load particular items with
custom fields. With this commit you can do it directly when loading
item by ID:

```
file = client.file_by_id('12345', [:name, :version_number])
file.version_number
```

Or manually by calling `#update_metadata`:

```
file = client.file('document.pdf')
file.reload_meta([:version_number])
file.version_number
```

You can find more information here: https://developers.box.com/docs/#fields