Fixes issues with $.ajax and shortcut functions

This fixes some issues with the functions $.ajax, $.get, $.getJSON and $.post:
- The handle to the success function was not passed to $.ajax if the *data* parameter was provided.
- The shortcut functions expected the *data* parameter to be an object, although $.ajax needs an url encoded string.
- $.post should be able to POST *application/x-www-form-urlencoded* data type, as PHP requires it to set the $_POST array (http://ch1.php.net/manual/en/reserved.variables.post.php#114598).