Add hover styles to close icons

**Context**: Please refer to my recently closed pull request on atom/tabs: https://github.com/atom/tabs/pull/27

In response to atom/tabs changing the cursor of the close button on tabs, I've made some adjustments to this theme that will make closing tabs more intuitive.

Very bad quality gif:

![Screen Cast](https://f.cloud.github.com/assets/1610503/2360523/7cf68db2-a625-11e3-9d54-92d3e1d6132b.gif)