Style book navigation/menu links

Navigation menu is now set to display as block:

.book-navigation ul.menu,
.book-navigation ul.menu li {
  display: block;
}

Now need to add collapse/expand list stlye.

Changed from above to:

.book-navigation ul.menu {
  display: block;
  padding-bottom: .5em;
}

To output list images - having li display as block prevented them from displaying.

Also added padding bottom.