Add Configuring tasks link to the header nav bar

Note that I slightely changed the icons:

![screen shot 2014-09-25 at 14 45 36](https://cloud.githubusercontent.com/assets/301201/4409303/13f54636-44dc-11e4-982e-f9f02db7daa5.png)

It partially fixes the #133 commit.

Still missing the API link on the documentation sidebar, which will come in a next commit.