Fix bug on the example, when signing up with email

On the example, when you sign up a new user without using an email, everything
works fine. However, if you do define an email address an exception is raised.

The reason why this happens, is that no email server has been setup. The
proposed fix is to use Django's console email backend. There are other solutions
but this one seems to be the simplest.  A tested alternative has also been added
in the documentation.

In order to avoid any pitfalls a warning has been added to remove the console
email backend and setup a proper email server.

``` python

Environment:

Request Method: POST
Request URL: http://127.0.0.1:8000/accounts/signup/

Django Version: 1.5.4
Python Version: 2.7.5
Installed Applications:
('django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.sites',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'django.contrib.admin',
 'allauth',
 'allauth.account',
 'allauth.socialaccount',
 'allauth.socialaccount.providers.dropbox',
 'allauth.socialaccount.providers.facebook',
 'allauth.socialaccount.providers.google',
 'allauth.socialaccount.providers.github',
 'allauth.socialaccount.providers.linkedin',
 'allauth.socialaccount.providers.openid',
 'allauth.socialaccount.providers.persona',
 'allauth.socialaccount.providers.soundcloud',
 'allauth.socialaccount.providers.stackexchange',
 'allauth.socialaccount.providers.twitch',
 'allauth.socialaccount.providers.twitter',
 'allauth.socialaccount.providers.vimeo',
 'allauth.socialaccount.providers.weibo',
 'example.demo')
Installed Middleware:
('django.middleware.common.CommonMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware')

Traceback:
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  115.                         response = callback(request, *callback_args, **callback_kwargs)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/django/views/generic/base.py" in view
  68.             return self.dispatch(request, *args, **kwargs)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/allauth/account/views.py" in dispatch
  43.                                                                     **kwargs)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/allauth/account/views.py" in dispatch
  99.                                                           **kwargs)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/django/views/generic/base.py" in dispatch
  86.         return handler(request, *args, **kwargs)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/django/views/generic/edit.py" in post
  165.             return self.form_valid(form)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/allauth/account/views.py" in form_valid
  130.                                self.get_success_url())
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/allauth/account/utils.py" in complete_signup
  149.                          signal_kwargs=signal_kwargs)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/allauth/account/utils.py" in perform_login
  113.             send_email_confirmation(request, user, signup=signup)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/allauth/account/utils.py" in send_email_confirmation
  276.                                                     signup=signup)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/allauth/account/models.py" in send_confirmation
  54.         confirmation.send(request, signup=signup)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/allauth/account/models.py" in send
  125.                                 ctx)
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/allauth/account/adapter.py" in send_mail
  87.         msg.send()
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/django/core/mail/message.py" in send
  255.         return self.get_connection(fail_silently).send_messages([self])
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/django/core/mail/backends/smtp.py" in send_messages
  88.             new_conn_created = self.open()
File "/home/faramir/.virtualenvs/bug/lib/python2.7/site-packages/django/core/mail/backends/smtp.py" in open
  49.                                            local_hostname=DNS_NAME.get_fqdn())
File "/usr/lib64/python2.7/smtplib.py" in __init__
  250.             (code, msg) = self.connect(host, port)
File "/usr/lib64/python2.7/smtplib.py" in connect
  310.         self.sock = self._get_socket(host, port, self.timeout)
File "/usr/lib64/python2.7/smtplib.py" in _get_socket
  285.         return socket.create_connection((host, port), timeout)
File "/usr/lib64/python2.7/socket.py" in create_connection
  571.         raise err

Exception Type: error at /accounts/signup/
Exception Value: [Errno 97] Address family not supported by protocol

```