Adds Joomla Ajax Interface to Joomla core

Feature tracker item at http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8549&tracker_item_id=31800

Details:
This adds an extensible solution to provide Ajax support in core. It acts as an entry point for Ajax functionality in Joomla, designed to execute plugins following the onAjax[foo] naming convention or can return data from a [foo]Ajax method in a module's helper.php file. The goal is to provide an easier way to provide Ajax support where it doesn't already exist.
 
The source project for this extension can be found at https://github.com/betweenbrain/Joomla-Ajax-Interface with an exmaple module at https://github.com/betweenbrain/Ajax-Session-Module.
 
Related Joomla list thread at https://groups.google.com/d/msg/joomla-dev-general/i1syYWshGsY/3udixCaRnRAJ