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