MLHR-749 #resolve #comment Updated style for notifications history pane

Two changes:
- reduce opacity of history pane when not hovering
- centered over mem usage gauge

![screen shot 2014-02-13 at 5 15 08 pm](https://f.cloud.github.com/assets/1390651/2167287/8c296906-9515-11e3-8071-391f947771f6.png)