Install scripts should check wheter the JSON module is installed

The function json_encode - which is provided by PHPs JSON-module - is used various times in the AJAX modules.

Due to the install-script not checking whether this module is installed, civicrm may show various errors and disfunctions during operation. I suggest to check for the json-module in the same way that we check for mysql.

Attached is a patch which applies to recent SVN-HEAD.