change div navigation to semantic html header tag

[lilmubby]

changed the div with the id 'nav' which wraps around navigation items to a header>nav>ul>li+a, which is easy to read and debug
example

-<div id='nav'> <div><a><a/></div></div>
+<header><nav><ul><li><a></a></li></ul></nav></header>

added a custom value to all div elements. Included the custom class name in the div reset CSS declaration