PHP XTemplate Code

* Added XTemplate::comment_delim defaults to '#'
* Added XTemplate::callback_delim defaults to '|'
* Tag callback functions - improve security (thanks Oliver):
* Added XTemplate::allow_callbacks property (boolean) to allow switching functionality off
* Added XTemplate::allowed_callbacks (array) to limit functions available from templates
* Callbacks now only available for methods of subclasses of XTemplate or as functions in XTemplate::allowed_callbacks array