Two additional user groups - Banned & Custom

Addition of two extra user groups, a Banned group which is treated the
same as an individual chat ban, except the ban does not expire, plus a
Custom user group which is treated the same as the standard
AJAX_CHAT_USER group. What good is this for? Well maybe you want to
distinguish a special group of chatters from the rest with their own
username colour - e.g. top contributors, help team, 'paid' membership
perks...

Use of both the banned and custom groups is configured via your forum's
specific CustomAJAXChat.php file.
Custom user group style for grey.css
Custom user group now uses different text colour

Plus one edit to chat.js to give the new AJAX_CHAT_CUSTOM user group the
ability to delete their own messages, just like AJAX_CHAT_USERS is able
to.
Merge pull request #138 from marquisite/testing

Two additional user groups - Banned & Custom