Validate email address case insensitive uniqueness at the model level

[jpotterm]

EmailAddress.email is currently treated as case insensitive unique on login, but that is only enforced in the signup form. In order to enforce that in the admin and when someone programmatically adds an email address it needs to enforce it at the model level. That's what this pull request does.

[coveralls]

Coverage decreased (-0.2%) to 69.151% when pulling c82b67e on jpotterm:email_validate_unique_case_insensitive into 1a5cab5 on pinax:master.

[coveralls]

Coverage decreased (-0.2%) to 69.151% when pulling c82b67e on jpotterm:email_validate_unique_case_insensitive into 1a5cab5 on pinax:master.

[coveralls]

Coverage decreased (-0.2%) to 69.151% when pulling c82b67e on jpotterm:email_validate_unique_case_insensitive into 1a5cab5 on pinax:master.

[coveralls]

Coverage decreased (-0.2%) to 69.151% when pulling c82b67e on jpotterm:email_validate_unique_case_insensitive into 1a5cab5 on pinax:master.

[brosner]

I am in favor of this change, but I'd like for this to get some test coverage before I merge.

[jpotterm]

@brosner I've added a test case for this.