All code handling email addresses has been update to use a new function
normalise_email, to have one consistent way of handling email addresses.

This function lowercases the domain part of an email address if it can
find an @, so it works on partial addresses as well.

A basic security measure to ensure a user is aware if his/her account
gets compromised.  The email contains a link to the password reset form
which allows a new password to be set.

Fixes #436

* The views check that the guest email address has been entered.
* The email address is written out against the order.
* An email confirmation is sent to the guest user email address.