fixed topbar button hover background issue

when the mouse is over a li in the topbar that contains a button but not over the button that changes background-color while not expected to change.
This should fix the problem.
![topbar-button](https://f.cloud.github.com/assets/3147882/1334813/255a8edc-35af-11e3-93d5-7dab070a0ff1.png)