add(format, levelId, callback) {
		switch (format) {
			case "tmx":
				// just load the level with the XML stuff
				if (levels[levelId] == null) {
					//console.log("loading "+ levelId);
					levels[levelId] = new TMXTileMap(levelId, getTMX(levelId));
					// level index
					levelIdx.push(levelId);
				} else {
					//console.log("level %s already loaded", levelId);
					return false;
				}

				// call the callback if defined
				if (callback) {
					callback();
				}
				// true if level loaded
				return true;

			default:
				throw new Error("no level loader defined for format " + format);
		}
	}
add a level into the game manager (usually called by the preloader)
@name add
@memberof level
@public
@param {string} format - level format (only "tmx" supported)
@param {string} levelId - the level id (or name)
@param {Function} [callback] - a function to be called once the level is loaded
@returns {boolean} true if the level was loaded