Separate karma from user's name with parens

Minor cosmetic fix. Surrounds karma points with parens, to separate it from the user's name.

Changes in the following places:

* Nav link in header
* Users list page

![screen shot 2014-05-19 at 12 11 57 pm](https://cloud.githubusercontent.com/assets/84005/3010901/c590457a-df20-11e3-9de5-9e30a3a08fab.png)