Add basic navbar generator. HTML and Bootstrap v3 options

[ericfernance]

Hi Mate,

I'm not sure if you wanted to add this into Burgers or whether you would prefer me just to bundle as a separate plugin. The only reason I thought of including in Burgers is that it uses the Burger's users and groups database tables.

This PR will add a new class to Burgers \Nav. This class will build a list of items, or if using \Nav::render('bootstrap') will add the necessary classes to support Bootstrap style drop down menus.

I use it with Bootstrap, hence the reason it's only Bootstrap at present.

When building the menu items to include, the Nav class checks the user's groupId is greater than or equal to the access_level defined in the menus table. I didn't have huge amounts of flexibility in the column naming as I was writing code to suit an already defined and running database.

It's up to you if you want to include it. It's pretty tightly linked to Burger's users and groups. I also thought it may complement some of the tools that are already there to help fatten up F3 a little.

Cheers,
Eric.