Username case inconsistencies

This patch lets users log in with "username" even if they signed up with "uSeRName".

The ```clean_username``` function in ```BaseSignupForm``` won't let users sign up with a username if it matches another regardless of case, so it makes sense that the user should also be able to log in with any combination of case.
See: https://github.com/pennersr/django-allauth/blob/master/allauth/account/forms.py#L204

A settings variable ```ACCOUNT_USERNAME_CASE_SENSITITIVE``` defaulting to ```False``` might be useful to people who want to have usernames case sensitive. I'm happy to submit a pull request for it if there's interest.

Thanks!
-Gus