Add 'on the fly' cache adding support

Enable to effortlessly cache an item when it's not already present :

example :

	memoryCache.get('myKey', function foudCallback(myData) {
		// my data has been found or generated !
		// use it
		doSomething(myData);

	}, function notFoundCallback(key, storeItCallback) {
		// my data was not found
		// generate it
		var myData = 'I am data';
		// trigger the callback,
		// this will store the generated item in cache
		// and pass it to 'foudCallback'

		storeItCallback(myData, 60 * 60 * 24);
	});