title: Django Money - Add two MoneyFields in different currencies and generate output in given currency
body: <p>Hi I've been scratching my head over past few days to figure out how to manipulate dj-money fields.
  Here are my two model fields: </p>
  
  <pre><code> client_cost = MoneyField(
      _('client cost'),
      max_digits=10,
      decimal_places=2,
      default_currency='AUD',
      null=True,
  )
  
  camera_operator_cost = MoneyField(
      _('camera operator cost'),
      max_digits=10,
      decimal_places=2,
      default_currency='AUD',
      null=True,
  )
  </code></pre>
  
  <p>They can have completely different currencies (default being 'AUD'). </p>
  
  <p>I need to be able to: </p>
  
  <ul>
  <li>extract currency from these fields. (Need the exact statement).</li>
  <li>add these two fields which could have amounts in any currency.  </li>
  <li>reformat the total in whatever currency I want. Let's say total in AU$ is 250 and now i want the same value in US$ which will be ~184US$</li>
  </ul>
  
  <p>Please help.</p>
  
  <p>PS: I'm using this: <a href="https://github.com/django-money/django-money" rel="nofollow noreferrer">https://github.com/django-money/django-money</a></p>