Make django-allauth's login requirements more strict

sigh. I can't get a good combination of django-allauth's configuration
parameters to work properly:

  http://django-allauth.readthedocs.org/en/latest/configuration.html

With ACCOUNT_USERNAME_REQUIRED and ACCOUNT_EMAIL_REQUIRED set to False,
social login is much smoother (one-click login) but normal email-based
sign-up is problematic: you when registering you only get the boxes:

  Email (optional):
  Password:
  Password confirmation:

... which is confusing - you get assigned a username (e.g. 'user').  If
you then set ACCOUNT_EMAIL_VERIFICATION = True, then the email
confirmation when you use Twitter, say, goes to an address that I don't
know where it comes from (!), e.g. my work email address, which isn't
associated with my Twitter email address in any way.

I suspect the problem will be more malicious users than people being put
off by sign-up, so this makes everything stricter - you have to pick a
username, an email address and confirm that email address even if you're
using social login.

In fact, my feeling is that we ought to just ditch the social login and
stick with email-based sign-up.
Remove old remarks from the README