Percent-encode the addressbook URL so it can contain special and national characters, bug #2571

Needed if one wants to have email addresses for usernames, because some servers require adddress book URLs to contain the username exactly as it is, and the '@' character must be escaped in an URL according to RFC 3986 section 2.1 http://tools.ietf.org/html/rfc3986#section-2.1

Thunderbird's URL parser can distinguish between uses of '@' as embedded authentication vs part of an URL path segment in simple cases, but not all.

Reported in http://www.sogo.nu/bugs/view.php?id=2571