Adding fade option to gumby.tabs.js

Adding fade effect to tabs by adding class `fade` to the section `tabs`

````javascript
Tabs.prototype.click = function($this) {
		// index of item to activate
		var index = $this.parent().index();
        var fade = this.$content.parent().hasClass('fade');
        var activecontent = this.$content.eq(index);
        var lastactivecontent = this.$content.parent().find('.tab-content.active');

		if(this.$nav.eq(index).add(this.$content.eq(index)).hasClass('active')) {
			return;
		}

		Gumby.debug('Setting active tab to '+index, this.$el);

		// deactivate other tab navigation and content
		this.$nav.removeClass('active');
		// activate this tab nav link and content
        this.$nav.eq(index).addClass('active');
        //fade effect
        if(fade == true){
            $(lastactivecontent).fadeOut(200,'swing',function(){
                $(this).removeClass('active').removeAttr('style');
            });
            $(lastactivecontent).promise().done(function(){
                $(activecontent).fadeIn(200,"swing",function(){
                    $(this).addClass('active').removeAttr('style');
                });
            });
        }
        else {
            this.$content.removeClass('active');
            this.$content.eq(index).addClass('active');
        }
		// trigger gumby.change event and pass current active tab index
		Gumby.debug('Triggering onChange event', this.$el);
		this.$el.trigger('gumby.onChange', index);
	};
````