Deprecate ajax "global" event behavior

[mikesherov]

Currently the ajax events are "global" events in that they will fire on any element, e.g., $(".anything").ajaxStart(fn). The element doesn't even need to be attached to the DOM! This requires us to do some expensive bookkeeping and special-case code.

As of jQuery 1.8, the docs should encourage users to attach ajax events only to document. In 1.9 we can have them only fire on document, with a plugin for backcompat.

see http://bugs.jquery.com/ticket/11717 for more info