title: Get refresh token for Box provider
body: <p>I'm using Boxr gem
  <a href="https://github.com/cburnette/boxr" rel="nofollow">boxr</a> for my autotests I need to do some manipulation with files and I have created developer token and did some things, after some time developer token was expired. 
  BUT on documentation I saw that I can do refresh token before runing tests.</p>
  
  <p><strong><em>If you want Boxr to automatically refresh the tokens once the access token becomes invalid you can supply a refresh token, along with your client_id and client_secret, and a block that will get invoked when the refresh occurs.</em></strong></p>
  
  <pre><code>token_refresh_callback = lambda {|access, refresh, identifier| some_method_that_saves_them(access, refresh)}
  client = Boxr::Client.new('zX3UjFwNerOy5PSWc2WI8aJgMHtAjs8T',
                            refresh_token: 'dvfzfCQoIcRi7r4Yeuar7mZnaghGWexXlX89sBaRy1hS9e5wFroVVOEM6bs0DwPQ',
                            client_id: 'kplh54vfeagt6jmi4kddg4xdswwvrw8y',
                            client_secret: 'sOsm9ZZ8L8svwrn9FsdulLQVwDizKueU',
                            &amp;token_refresh_callback)
  </code></pre>
  
  <p>The question is: What the method should be for save refresh token? <strong>"some_method_that_saves_them"</strong></p>