Use theme colors for the status line

This change makes the status line use the same theme colors as the zoom percentage drop down that's in the bottom left corner of the editor.

![diff](https://cloud.githubusercontent.com/assets/710598/4514736/923a6ae0-4b89-11e4-8272-2f57ec2cead8.png)

At first I used the `ToolWindowTextBrushKey` and `ToolWindowBackgroundBrushKey` colors, which looked OK, but I think it looks best using the same colors as the zoom percentage drop down (even though this isn't a drop down). I didn't spend a lot of time looking through the various other colors provided by  [`EnvironmentColors`](http://msdn.microsoft.com/en-us/library/microsoft.visualstudio.platformui.environmentcolors(v=vs.110).aspx), so it's possible there may be more appropriate colors that I missed.

I specified these in code using `SetResourceReference` and not in XAML (like [this](http://stackoverflow.com/questions/18913208/colors-in-visual-studio-extension)) because I'm not familiar enough with VS extensions to know whether a `clr-namespace` import of `Microsoft.VisualStudio.PlatformUI` for assembly `Microsoft.VisualStudio.Shell.11.0` will work in both VS 2012 and VS 2013. Upgrade instructions like [this](http://msdn.microsoft.com/en-us/library/jj991932.aspx) makes me think it's not possible to do in XAML (without some kind of ifdef), because it says to change `Microsoft.VisualStudio.Shell.11.0` to `Microsoft.VisualStudio.Shell.12.0` when going from 2012 to 2013.

Tested with the Blue, Dark, and Light themes in VS 2013. I don't have VS 2012 installed, so haven't tested these changes there.