Registers an option to the pool of options.

@param {object} data
@return {boolean|string}
function(data) {
		var key = hash_key(data[this.settings.valueField]);
		if (typeof key === 'undefined' || key === null || this.options.hasOwnProperty(key)) return false;
		data.$order = data.$order || ++this.order;
		this.options[key] = data;
		return key;
	}