[PoC] Add jQuery.migrateWarnCallback feature

[kavu]

The reason why I added this, is that with such callback I can notify some exception tracking service with deprecation messages. For example — notifying rollbar.com:

$.migrateWarnCallback = function(msg) {
  _rollbar.push({
    level: 'error',
    msg: "JQMIGRATE: " + msg
  });
};

Thanks!

[dmethvin]

The facilities are already there to do this, using a timer. That has the advantage of sending the bulk of the messages in one shot, since many of them will happen on initialization. So after document ready just set a timer for, say, every 5 seconds and check whether new messages have been put into the warnings array.