Added commented simplenav tags to global.html for the generator and dumm...

This could be potentially controversial since it's something more specifically for koi, but would be super handy to have there so we don't need to look up or remember the tag. 

I've added in a simplenav tag in to the header (targeting key: header_navigation, levels 1+2), mobile tray (header_navigation, levels 1+2) and footer (footer_navigation, level 1)

I have commented them out by default so the dummy app doesn't throw errors. 
Let me know what you think :)