You just over complicated things. Get rid of all the positions you have used because you do not need them at all. I just mocked something up on codepen for you to have a look: http://codepen.io/nighrage/pen/mhgze. The following css is all you need and the menu will be responsive to a certain extend. Let me know if you want to do something in addition to how it is displayed.
#nav ul {
list-style-type:none;
margin:0;
padding:0;
width:100%;
float:left;
text-align:center;
}
#nav ul li {
display:inline-block;
padding: 0 10px;
}