Better alignment for nested lists (fixes #25)

Currently, nested lists all align on the same left margin. Making them
nest properly was as simple as an alteration on their left-margin CSS
property. Hope that's sufficient.

Before: ![before](https://dl.dropbox.com/u/94814/Slingshot/Pictures/Screen%20Shot%202012-08-03%20at%2010.24.48%20AM.png)

After: ![after](https://dl.dropbox.com/u/94814/Slingshot/Pictures/Screen%20Shot%202012-08-03%20at%2010.25.04%20AM.png)

Signed-off-by: David Celis <david@davidcelis.com>