Quoting RFC: "However, exploiting the case sensitivity of mailbox
local-parts impedes interoperability and is discouraged."

Most of the existing software is threating the local part (i.e. the
thing before @) of emails as case insensitive.

Some people use different capitalization at different times, e.g.
arkadiusz.hiler@example.com with git-send-email and
Arkadiusz.Hiler@example.com with their regular client.

To mitigate this Person's email is now case insensitive for the sake of
searching and normalized to lower case pre_save.

It's not perfect by any means and still can be bypassed by doing
.update() on the QuerySet, but will work in most of the usual cases.

Tests and migration deduplicating existing entries are provided.

Fixes: #32


Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>