Add ability to set namespace globally via js. 

Add ability to set namespace globally via js. 
Use global namespace if it has been set otherwise default to meta.foundation-data-attribute-namespace. 

I ran into some issues when the foundation sass project was a tiny version behind the .js project. the .foundation-data-attribute-namespace class name did not exist in my version of the foundation sass project. Which caused the js to make poo. Setting Foundation.global.namespace = 'my-namespace' would get overridden by 'sans-serif', since I didn't have the .foundation-data-attribute-namespace class defined. This pull request ensures that the .js global.namespace property always takes precedence. 

So now one can do something like: 

    <div class="reveal-modal" data-my-namsespace-reveal>Super awesome modal</div>

....

    <script>
        Foundation.global.namespace = 'my-namespace';

        // Go go gadget, reveal!
        $(document).foundation( 'reveal' );
    </script>