com_ajax: improvement, use controllers, better error handling

can be a bit late, but but...

this pull change com_ajax to work in Joomla! way, and work like described http://docs.joomla.org/Xml-rpc_changes_in_Joomla!_1.6
here each format have a own controller and each "element" (plugin, module) have own controller.
It allow more simple extend this component in future, if will be need

call link changed to:
for plugins:
`index.php?option=com_ajax&task=plugin.call&name=foo&format=json`
`index.php?option=com_ajax&task=plugin.call&name=foo&format=raw`

for modules:
`index.php?option=com_ajax&task=module.call&name=foo&format=json`
`index.php?option=com_ajax&task=module.call&name=foo&format=raw`

Another formats we can add in future, by request.
Other changes:
 - response format from #1960 by @piotr-cz
 - `format` parameter now is required
 - own Exception handler that should fix previous problem with handling errors when "ajax" call,
 - own function for check "whether module available"
 - added `content` and `system` plugin groups

links:
 - related pull #1960 and #1778 
 - Joomla! tracker link will be soon