Fix submenu for small width (#62)

Issues: 

1. In desktop, the hovering drops the right-pointing triangles to the middle (should be top);
2. Touch-only devices cannot toggle the expansion of sub-menu items at all:

Both are fixed. 2 was cause by `<a>` not having href attribute.

As for the bootstrap suggestion, I don't see one easily enough to implement. I'd tend to think if there is, it would be included in the official docs. As I previously mentioned, it [has been discussed in bootstrap](https://github.com/twbs/bootstrap/pull/6342) about the reason for dropping support of submenus, and also a suggested method(which is pretty much what we have here.)

So.. still...
![cooool](https://f.cloud.github.com/assets/1153134/1012524/83c87f14-0b71-11e3-8202-d569d3cf671c.png)