Bug 10402: Use an object for contacts

Bug 10402: Use an object for contacts

In preparation for adding the ability to handle multiple contacts, this
patch moves booksellers' contacts into their own class,
C4::Bookseller::Contact.

To test:
1) Apply patch.
2) Run database update.
3) Edit a bookseller, making sure to add a contact.
4) View the bookseller's information, making sure the contact
   information is there.
5) Run the unit test:
    > prove t/db_dependent/Bookseller.t

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Paola Rossi <paola.rossi@cineca.it>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>