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) Edit a bookseller, making sure to add a contact.
3) View the bookseller's information, making sure the contact
   information is there.
4) Run the unit test:
    > prove t/db_dependent/Bookseller.t
Bug 10402: Move contacts to separate table

This patch normalizes the data structures used for bookseller
contacts.

To test:
1) Repeat tests described on previous patch.
Bug 10402: Add ability to enter multiple contacts

Some vendors may have more than one contact. For example, a technical
contact and a billing contact, or a contact for journals and a contact
for monographs. Rather than require that each contact be either made
into a separate vendor or recorded somewhere outside of Koha, it would
be really useful of Koha had the ability to add multiple additional
contacts to vendors in the Acquisitions module.

To test:
1) Apply patch.
2) Edit a bookseller, making sure to add a contact.
3) View the bookseller's information, making sure the contact
   information is there.
4) Run the unit test:
    > prove t/db_dependent/Bookseller.t
5) Add multiple contacts to a vendor, see that they show up.
6) Delete one contact from a vendor with multiple contacts,
   see that the result is correct.
7) Sign off.

Note: This test plan can supersede that on the previous two patches,
as all functionality of the previous two patches is required by this
one.