title: Issues with username field in Python-social-auth
body: <p>I am using Python socia auth for face-book. I have modified default Django behavior of User Model and removed <code>username</code> field .</p>
  
  <p>I have added this in custom user model : <code>USERNAME_FIELD = 'email'</code></p>
  
  <p>BUt I am getting this error when trying to login </p>
  
  <pre><code>TypeError at /complete/facebook/
  'username' is an invalid keyword argument for this function
  </code></pre>
  
  <p>I know when it is trying to create user it doesn't find username field and this throwing this error.</p>
  
  <p>I have defined below settings but still my issue remains as it is : </p>
  
  <pre><code>SOCIAL_AUTH_USER_MODEL = 'accounts.User'
  SOCIAL_AUTH_USERNAME_IS_FULL_EMAIL = True
  </code></pre>
  
  <p>Any solution for this ?</p>