Allow step.element to use function

Step "element" can now use a function to dynamically select the DOM element for the step. Usage:

element: function() { return $(document.body).find(".my-dynamic-element"); },
title: "Dynamic!",
content: "This element was found by jquery",

Change to Tour.prototype.getStep = function (i)

build/js/bootstrap-tour.js

@@ -107,6 +107,10 @@ var bind = function (fn, me) {
 
 		Tour.prototype.getStep = function (i) {
 			if (this._options.steps[i] != null) {
+
+				if(typeof(this._options.steps[i].element) == "function")
+					this._options.steps[i].element = this._options.steps[i].element();
+
 				return $.extend({
 					id: "step-" + i,
 					path: '',