Make tab bottom border positioned relative to bottom

Both this theme and the light theme have the tab bar's underline positioned assuming the tab bar is the only thing in the top section.  Anything added using atom.workspaceView.prependToTop will cause layout issues: 

![screen shot 2014-12-22 at 11 23 02 pm](https://cloud.githubusercontent.com/assets/343042/5534656/7ee06afa-8a34-11e4-80c4-b3b8f2dd9ac2.png)