title: django-import-export: cannot exclude id field during import : KeyError: u'id'
body: <p>On Django-1.9.6, django-import-export-0.5</p>
  
  <p>When I try to upload CSV without "id" field throws this error. </p>
  
  <pre><code>Line number: 1 - u'id'
  13173474, Harry McDade, 10.harry.asas@asasasas.com
  Traceback (most recent call last):
  File "/Users/isanka/dev/venv/edxubase/lib/python2.7/site-packages/import_export/resources.py", line 434, in import_row
  instance, new = self.get_or_init_instance(instance_loader, row)
  File "/Users/isanka/dev/venv/edxubase/lib/python2.7/site-packages/import_export/resources.py", line 258, in get_or_init_instance
  instance = self.get_instance(instance_loader, row)
  File "/Users/isanka/dev/venv/edxubase/lib/python2.7/site-packages/import_export/resources.py", line 252, in get_instance
  return instance_loader.get_instance(row)
  File "/Users/isanka/dev/venv/edxubase/lib/python2.7/site-packages/import_export/instance_loaders.py", line 31, in get_instance
  field = self.resource.fields[key]
  KeyError: u'id'
  </code></pre>