UNUSABLE_PASSWORD was removed in Django 1.6

Now we have to use an is_password_usable function
that works with both Django versions. We can't use
django.contrib.auth.hashers.is_password_usable either, because it not
only checks against the unusable password, but checks for a valid hasher
too. We need different error messages in those cases.

See Django ticket #20079.
New password reset uses base 64

Django ticket #14881 switched to base64 encoding user ids instead of the
old base36. This patch makes our view work with either format.
Test on Django 1.6

Stop testing against Django master, because it's very much in flux right
now. We can resume testing against master once it settles down.
reference the changes to Django 1.6's password_reset_confirm view
Django 1.6 support

* django1.6:
  reference the changes to Django 1.6's password_reset_confirm view
  Test on Django 1.6
  New password reset uses base 64
  UNUSABLE_PASSWORD was removed in Django 1.6

Conflicts:
	CHANGES
The PasswordResetCompleteView wasn't calling resolve_url on login_url.

This was making it impossible to use a named url as your LOGIN_URL
setting.