Fixes facebook response.status values and adds callback support for cancelUrl and errorUrl

response.status bugfix
---------------------------------

It seems that the ``notConnected`` status disappeared and now Facebook uses ``connected``, ``not_authorized`` and ``unknown`` as possible values https://developers.facebook.com/docs/reference/javascript/FB.getLoginStatus/

In case of ``not_authorized`` and ``unknown``, the ``cancelUrl`` should be used.

Callbacks support
-------------------------

This pull request adds a new feature, that is the possibility to use a callback function for ``cancelUrl`` and ``errorUrl``.

In other words, you can create ``templates/facebook/fbconnect.html`` and use custom logic in case of facebook cancel or error:


	{% load url from future %}
	{% load staticfiles %}
	<div id="fb-root"></div>
	<script type="text/javascript" src="{% static "facebook/js/fbconnect.js" %}"></script>
	<script type="text/javascript">
	allauth.facebook.init({ appId: '{{facebook_app.client_id}}',
	  locale: '{{facebook_jssdk_locale}}',
	  loginOptions: {{fb_login_options}},
	  loginByTokenUrl: '{% url 'facebook_login_by_token' %}',
	  channelUrl : '{{facebook_channel_url}}',
	  cancelUrl: function() {   # <== new
	  	// don't do anything
	  },
	  logoutUrl: '{% url 'account_logout' %}',
	  errorUrl: function() {   # <== new
	  	alert('An error occurred while attempting to login via Facebook');
	  },
	  csrfToken: '{{csrf_token}}' });
	</script>