Added hover background and click to sidebar nav

This is an extension of #1314.

This PR changes the sidebar nav highlighting from an underline to a background colour change, as well as making the menu item clickable across it's full width (vs just the text/icon boundaries). 

![screenshot 2014-07-12 00 20 57](https://cloud.githubusercontent.com/assets/429311/3560048/46d6cf30-0958-11e4-80a4-2a5b6f8f35b1.png)

Is there a reason to expect other `.task-link-group` classes to exist, necessitating the `#tasks` selector?