Add support for online address books using the CardDav format.

User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7 Build Identifier: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.1.7) Gecko/20100111 Thunderbird/3.0.1 Apple has a new Address book server which is based upon the new CardDav protocol. This is very similar to the CalDav protcol supported by Sunbird. It would be great if Thunderbird supported this type of address book. Reproducible: Always Actual Results: Support doesn't exist Expected Results: Support existed