Add dark-theme for gtk+3

In Ubuntu, the default theme is dark gray, so it is a nice match for the Atom dark default theme. However, in gnome-3 the default theme is white and it kind of contradicts with Atom dark theme.  Here is how it looks:

![atom-gnome-dark-1](https://cloud.githubusercontent.com/assets/6003742/5425488/d428c002-82cd-11e4-930e-1688794001ba.png)

Since atom-shell is now supporting [dark theme in gtk3+](https://github.com/atom/atom-shell/blob/master/docs/api/browser-window.md#class-browserwindow) I thought it would be nice to have the dark theme by default, however there is an open issue [#765](https://github.com/atom/atom-shell/issues/765) in atom-shell where the menu bar will still be white while the top bar is black.

I am going to leave this here so we can review it, and once the issue is resolved, you may want to merge it.

Thank you.