Added new setting, widget, form and auth backend to allow login via user email address

To enable:
AUTHENTICATION_BACKENDS = ('common.auth.email_auth_backend.EmailAuthBackend',)
COMMON_LOGIN_METHOD = 'email'

apps/common/auth/email_auth_backend.py

@@ -0,0 +1,23 @@
+# From: http://www.micahcarrick.com/django-email-authentication.html
+from django.contrib.auth.models import User, check_password
+from django.contrib.auth.backends import ModelBackend
+
+
+class EmailAuthBackend(ModelBackend):
+    """
+    Email Authentication Backend
+    
+    Allows a user to sign in using an email/password pair rather than
+    a username/password pair.
+    """
+
+    def authenticate(self, email=None, password=None):
+        """
+        Authenticate a user based on email address as the user name.
+        """
+        try:
+            user = User.objects.get(email=email)
+            if user.check_password(password):
+                return user
+        except User.DoesNotExist:
+            return None