Specify depth of navigation

I have been working on a project that has a mega menu and also a side menu.  Because of the duplicate navigation I only wanted the top navigation to show the first level of nav once its made into a select.  The alterations in this pull request add another settings key "depth" which allows the developer to set a specific depth to iterate to.

An example:

```js
$(".navigation_one > ul").tinyNav({
  active: 'current',
  depth: 2
});
```