Fix for 3rd level menu dropdowns.

This fix addresses a few fairly major issues I caught while setting up the theme using the WP Theme test data: http://codex.wordpress.org/Theme_Unit_Test

The 3rd level dropdowns are not styled at all, which I realize was the point, however in testing, they overlap the 2nd level dropdows, so I added styles to shift them 100% to the left.

The next issue is a specificity issue when using `.nav li:hover ul` which caused ALL sub menus to be visible when you hovered over the 1st level LI.  I updated this to `.nav li:hover > ul` to fix the issue.

The last issue was something I noticed on a whim,  After fixing the first two issues I noticed if you hovered over a lvl 2 item that had a sub menu, if you moved the mouse all the way to the left (don't hover over any other li's) and off the menu completely, the menu would hide, but the 3rd level items would remain stuck on the screen.

![test just another wordpress site](https://cloud.githubusercontent.com/assets/1275573/2802671/cf5e0dd2-cc90-11e3-913c-11f64f456f67.png)

![untitled-1](https://cloud.githubusercontent.com/assets/1275573/2802661/b559e9e2-cc90-11e3-807b-0565d02be0ef.jpg)

Removing overflow:hidden/visibile; from the rules fixed the issue and at this time has not caused any side effects that I could find.