Fix pep8 issues

* Remove unused imports
* Remove superfluous url =
* Merge unnecessarily split lines
Convert email addresses to lower case

* Refactor email validation into ValidateEmailMixin
Coerce email address to lower case in admin
Re-add case insensitive check in validate_email

* Avoid creating a duplicate user with the same email address
Add CaseInsensitiveEmailBackend to AUTH_BACKENDS

* django.contrib.auth.authenticate now checks email address case-insensitively
Merge pull request #45 from incuna/lower_case_emails

Convert emails to lower case on User creation