improved style for TOC
- fix a.selected css selector into li.selected,
  since that class is applied to the <li> rather than the <a>
- improve overall styling (compare before: http://i.imgur.com/fHz8dMB.png
  and after: http://i.imgur.com/ub5QAaW.png)
- change alignment technique so it works regardless of font metrics
- minor grammar tweak to the extension's description string

Change-Id: I3884950719744bcbdb148fd9711ace823e993f74