contact/add - Creates a new contact.

Method parameters:
full_name - The full name of the contact.
alias - The alias of the contact.
address - The address of the contact.
email1 - The primary email of the contact.
email1_label - The label of the primary email of the contact.
email2 - The secondary email of the contact.
email2_label - The label of the secondary email of the contact.
phone1 - The primary phone number of the contact.
phone1_label - The label of the primary phone number of the contact.
phone2 - The secondary phone number of the contact.
phone2_label - The label of the secondary phone number of the contact.
birthday_time - The Unix timestamp of the birthday of the contact.
username - The Zvini username of the contact.
timezone - The timezone offset of the contact in minutes.
tags - A space-separated list of tags.
notes - Additional notes of the contact.
favorite - Whether the contact should be marked as favorite.

Returns a JSON document of the following format:
<number> - The ID of the newly created contact.

Expected errors:
ENTER_FULL_NAME - The full name is empty.
TOO_MANY_TAGS - More than 5 tags given.