Sign In/Out CSS/HTML for #338

1. Includes the CSS and HTML markup for the [v4 sign in/out mockup](http://fullahead.org/work/wet-boew/3.1.0/sign-in-out/v4/content-signin_v4.png).
1. Hides #wb-session element for mobile view.
1. Doesn't include Spanish working examples (translations are needed).
1. Doesn't update mobile settings menu with new links but does include the following two CSS classes that can be used by the .js to build the mobile menu:

* `#wb-session .session`: sign in/out link.
* `#wb-session .settings`: 1 to many account setting links.

Let me know if you want me to tackle the mobile menu later in the week.