fixed facebook backend with new-style response returned from parse_qs

I was having trouble logging in with facebook on the requests branch in django 1.5.1 (see screenshot below) and realized that the problem was related to a change in how the `request` object in the facebook backend were handled now that `parse_qs` gets rid of lists with  `drop_lists`.

Its unclear to me whether other backends might be affected, but it looks like xing, stackoverflow, and yammer might have similar problems but I'm less confident in that:

    [term]$ cd social_auth/backends
    [term]$ grep "\[0\]" `find . -name "*.py" | xargs grep parse_qs | sed 's/:/ /' | awk '{print $1}' | uniq`
    ./contrib/xing.py:        profile = simplejson.loads(content)['users'][0]
    ./contrib/stackoverflow.py:            return data.get('items')[0]
    ./contrib/yammer.py:        email = response['user']['contact']['email_addresses'][0]['address']
    ./contrib/yammer.py:                data['code'] = extra['code'][0]


![Screen shot 2013-04-15 at 17 09 13](https://f.cloud.github.com/assets/255672/385674/197f3b38-a68c-11e2-809f-cf3fcda2be84.png)