Add ability to update currency 'factors' (exchange rates) from trusted source(s)

Having the ability to update existing currencies regularly with up-to-date values would make django-currencies one of those key apps for building improved e-commerce solutions.

Suggested methods of updating:
- manual method: post a CSV file to a specified django-currencies view to be parsed
- automatic method: cron job to retrieve values from a trusted 3rd party source (perhaps have modules for this: Bank of Canada, Bank of America, etc.)

One would likely need the notion of a base/default currency in order to drive this, however you likely already have this in mind in order to establish your base currency 'factors'.