@stylesheet tag breaks menu with 2 or more parents

### The Problem

When a `@stylesheet` page has more than one parent, the menu is rendered incorrectly:

What's incorrect is that the parent menu item ("Egowall") is rendering it's children.

What I've learned is that this due to the `@stylesheet` tag configuring a `hideChildrenInMenu` property:

``` js
this.hideChildrenInMenu = true;
```

https://github.com/bitovi/documentjs/blob/6978b85952f27bcbd7749a15198570a8e92f081d/lib/tags/stylesheet.js#L23

The `hideChildrenInMenu` property is designed to hide the children of the active `@stylesheet` page in the menu, so that they can instead be displayed as sub-headings within the page's content:

``` mustache
{{^if hideChildrenInMenu}}

{{makeTitle}}

{{> active-menu.mustache}}

{{/if}}
```

https://github.com/bitovi/documentjs/blob/6978b85952f27bcbd7749a15198570a8e92f081d/site/default/templates/menu.mustache#L14-L21

In reality, the property hides the active `@stylesheet` menu item itself as well as its children:

https://github.com/bitovi/documentjs/blob/6978b85952f27bcbd7749a15198570a8e92f081d/site/default/templates/menu.mustache#L16-L18

In order to add the active `@stylesheet` page back to the menu, the template renders the children of its parent menu:

```
{{#if ../active.hideChildrenInMenu}}
{{> active-menu.mustache}}
{{/if}}
```

https://github.com/bitovi/documentjs/blob/6978b85952f27bcbd7749a15198570a8e92f081d/site/default/templates/menu.mustache#L8-L10

Unfortunately, this renders the children of EVERY parent menu:

``` mustache
{{#each parents}}

{{makeTitle}}

{{#if ../active.hideChildrenInMenu}}
{{> active-menu.mustache}}
{{/if}}

{{/each}}
```

https://github.com/bitovi/documentjs/blob/6978b85952f27bcbd7749a15198570a8e92f081d/site/default/templates/menu.mustache#L3-L12
### A Fix

If you only show the LAST parent's child pages, everything works as was intended:

The way to accomplish this is to add a helper to check that a given item is the last in the the given list:

``` js
/**
* @function documentjs.generators.html.defaultHelpers.ifIsLastItem
*
* Renders the truthy section if the current context is the last item
* in the list.
*
* @param {Array} list
* @param {*} item
*/
ifIsLastItem: function(list, item, options){
return list[list.length - 1] === item ? options.fn(this) : '';
},
```

And add a condition to the rendering of child items in the parent menu:

``` mustache
{{#if ../active.hideChildrenInMenu}}
{{#ifIsLastItem ../../parents .}}
{{> active-menu.mustache}}
{{/ifIsLastItem}}
{{/if}}
```

If nobody has a problem with this solution, I'll submit a PR later this week.