Bug 16275: Do not allow a self registration with an existing email address

Bug 16275: Do not allow a self registration with an existing email address

To avoid multiple registrations, it would be good to check the unicity
of the primary email address.
This patchset adds a new pref PatronSelfRegistrationEmailMustBeUnique.
If on, a patron will get "This email address already exists in our
database" if he try to register with an existing email address.

Test plan:
1/ Register a new patron with an email address
2/ Make an other registration using the same email address
=> With the pref PatronSelfRegistrationEmailMustBeUnique on, you won't be allowed
=> With the pref off, no change should be noticed.

Sponsored-by: BULAC - http://www.bulac.fr/
Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Work as described, no errors.

Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>