title: Remove 'username' field from django-allauth
body: <p>When django-registration doesn't support django 1.5 and custom user model. I'm trying use django-allauth, from first look it's great product.</p>
  
  <p>Problem i have - username field required, but in my app i don't have username's. So, allauth documentation says:</p>
  
  <pre><code>**Available settings:**
  ACCOUNT_AUTHENTICATION_METHOD (="username" | "email" | "username_email")
  </code></pre>
  
  <p>Specifies the login method to use -- whether the user logs in by entering his username, e-mail address, or either one of both.</p>
  
  <p><strong>Ok, i done, and got error:</strong></p>
  
  <pre><code>AssertionError at /accounts/signup/
  No exception supplied
  </code></pre>
  
  <p><strong>models.py:</strong></p>
  
  <pre><code>class MyUser(AbstractBaseUser, PermissionsMixin):
      title = models.CharField ('Name', max_length=100)
      email = models.EmailField('Email', max_length=255, unique=True)
      ...
  </code></pre>
  
  <p><strong>settings.py</strong></p>
  
  <pre><code>ACCOUNT_USERNAME_REQUIRED = False
  ACCOUNT_AUTHENTICATION_METHOD = ('email')
  AUTH_USER_MODEL = 'internboard.MyUser'
  </code></pre>
  
  <p>What i'm doing wrong ?</p>