Add possibility to specify which fields should be loaded

[mrhead]

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

[mrhead]

I have issues running integration test suite. I am not sure what token should I put into account.yml. I tried developer token, access token, refresh token but neither of them worked and I always ended up with RubyBox::AuthError.

So any comments to this PR or help with integration testing are welcome.