Measures the width of a string within a
parent element (in pixels).

@param {string} str
@param {object} $parent
@returns {int}
function(str, $parent) {
	if (!str) {
		return 0;
	}

	if (!Selectize.$testInput) {
		Selectize.$testInput = $('<span />').css({
			position: 'absolute',
			top: -99999,
			left: -99999,
			width: 'auto',
			padding: 0,
			whiteSpace: 'pre'
		}).appendTo('body');
	}

	Selectize.$testInput.text(str);

	transferStyles($parent, Selectize.$testInput, [
		'letterSpacing',
		'fontSize',
		'fontFamily',
		'fontWeight',
		'textTransform'
	]);

	return Selectize.$testInput.width();
}