make EmailField return e-mail addresses in canonical form

The domain part in email addresses is case-insensitive by RFC, so x@foo.com is indistinguishable from x@fOo.cOm . This makes EmailField normalise the email representation by lower-casing the domain part. This fixes an issue with UNIQUE constraints on the model, where the constraint is not enforced if the user enters the same e-mail with different case representations.