Better Migrate support

This patch worked great for me. Thanks for contributing it!

Even though you've documented this in the code, it might be worth throwing an exception if someone tries to migrate country names that are longer than two characters (or perhaps better, are not valid ISO country codes). As it stands, you'll get SQL errors when trying to migrate "United States" in place of "US".

Tested and works for me, as well. Thanks for the patch :)

Also worked for me

Committing as is. I'm not sure where such an exception for invalid country codes would go in the Migrate class, but I'd be happy to patch that in if it's an accepted pattern for Migrate classes.

Not sure if this belongs in docs somewhere, but here's a suggested starting point for someone migrating into Address Field with country names in the source. This does Google geocoding since Google can understand typos in the country name, returning DE for "ermany", "Gerany", and other typos. In the migrate constructor use $this->addFieldMapping('my_destination_address_field', 'country_code');
The following takes inspiration from the Geocoder module, and requires it to be enabled with a Google API key entered.

    if (!empty($row->field_country)) {
      // Geocode the country name to the 2 letter country code expected by
      // Address Field.

      // Sleep between Google API requests to avoid being blocked by overuse.
      $delay_trigger = &drupal_static(__METHOD__);
      $delay = variable_get('geocoder_google_delay', 0);
      if ($delay > 0 && $delay_trigger) {
        usleep($delay * 1000);
      }

      // Get Google API key entered in the geocoder module.
      $geocoder_settings = variable_get('geocoder_settings', array());
      $query['key'] = $geocoder_settings['geocoder_apikey_google'];

      // Use "components" rather than an "address" to prevent Google from
      // mistaking an improperly typed country name as a city.
      $query['components'] = 'country:' . $row->field_country[0];
      $base_url = 'https://maps.googleapis.com/maps/api/geocode/json';
      $url = url($base_url, array(
        'query' => $query,
      ));

      // Send the request.
      $result = drupal_http_request($url);
      $delay_trigger = TRUE;

      if (isset($result->error)) {
        $args = array(
          '@code' => $result->code,
          '@error' => $result->error,
          '@uid' => $row->uid,
        );
        $msg = t('HTTP request to google API failed.\nCode: @code\nError: @error for user with source uid: @uid', $args);
        drush_print($msg);
        return FALSE;
      }

      $data = json_decode($result->data);

      if ($data->status == 'ZERO_RESULTS') {
        drush_print('zero results from geocoding country for source uid: ' . $row->uid);
        return FALSE;
      }
      elseif ($data->status != 'OK') {
        $args = array('@status' => $data->status, '@uid' => $row->uid);
        $msg = t('Google API returned bad status.\nStatus: @status for source uid: @uid', $args);
        drush_print($msg);
        return FALSE;
      }

      if (empty($data->results[0]->address_components[0]->short_name)) {
        drush_print('Empty country geocode result for source uid: ' . $row->uid);
        return FALSE;
      }

      $country_code = $data->results[0]->address_components[0]->short_name;
      $row->country_code = $country_code;
    }