Add a visual indicator that you haven't committed lately

## What is this?

This PR actually *uses* our visual extension to add a new icon to the corner of every editor window:

![image](https://f.cloud.github.com/assets/1396/474633/d3ad2a7a-b770-11e2-8242-e9b2f76acb07.png)

This icon changes colors and brightness depending on how long it has been since your last commit, traffic light style. Not committed in a long time? You'll see the angry Red commit icon above.

## Here's the cool part

Click that icon, and it'll automatically launch [GitHub for Windows](http://windows.github.com/) into the repo you're working on, with the commit box focused.

## TODO

- [x] Implement the traffic light logic, bind it up
- [x] Make the button look more button-like
- [x] Make sure we handle errors