[Engine-VBulletinChatbox] Initial vBulletin Chatbox support.

Initial protocol module for the vBulletin Chatbox plugin.
[Engine-VBulletinChatbox] Fix skipping old messages.
[Engine-VBulletinChatbox] Leave message parsing to MessageBuilder.
[Engine] Use HtmlAgilityPack to parse HTML and improve image parsing.
[Engine-VBulletinChatbox] Implement /me support.
[Engine-VBulletinChatbox] Trace calls.
[Engine-VBulletinChatbox] Update token if sending fails; Unicode

Update the security token and, failing that, try to log in anew if
sending a message fails the first time. Include commands to do this
manually.

Also, add Unicode support (decimal XML character escapes).