Fixed a timing edge case in the RelatedFieldWidgetSeleniumFirefoxTests.

By settings an implicit wait timeout for the find_* methods we can wait
till the javascript code add the new option to the DOM.

See http://ci.djangoproject.com/job/Django/database=mysql_gis,python=python3.3/3174/testReport/junit/admin_widgets.tests/RelatedFieldWidgetSeleniumFirefoxTests/test_foreign_key_using_to_field/

Stacktrace:
Traceback (most recent call last):
  File "/var/lib/jenkins/jobs/Django/workspace/database/mysql_gis/python/python3.3/tests/admin_widgets/tests.py", line 1060, in test_foreign_key_using_to_field
      self.assertEqual(username_value, new_option.get_attribute('value'))
      AssertionError: 'newuser' != 'testser'
      - newuser
      + testser