function _fnCallbackReg(settings, store, fn) {
		if (fn) {
			settings[store].push(fn);
		}
	}
Register a callback function. Easily allows a callback function to be added to
an array store of callback functions that can then all be called together.
 @param {object} settings dataTables settings object
 @param {string} store Name of the array storage for the callbacks in oSettings
 @param {function} fn Function to be called back
 @memberof DataTable#oApi