[fixed] Bug in Server vs Client side rendering of Navbar

Inconsistent rendering between the client and server primarily around
casing.

Closes react-bootstrap#475

Author: mtscout6

src/Navbar.js

@@ -32,7 +32,7 @@ const Navbar = React.createClass({
       bsClass: 'navbar',
       bsStyle: 'default',
       role: 'navigation',
-      componentClass: 'Nav'
+      componentClass: 'nav'
     };
   },