Bug 613551 — Add gContact namespace

Add support for various elements from the gContact namespace, and implement
them in #GDataContactsContact. There is now support for contact:
 * nicknames
 * birthdays
 * jots (notes about a contact)
 * relations (with other people, not necessarily family)
 * websites
 * events (anniversaries, important dates, etc.)
 * calendar links
Helps: bgo#613551