Add a visual indicator that you haven't committed lately

## What is this?

This PR actually *uses* our Margin extension to add a new icon to the corner of every editor window:

![image](https://f.cloud.github.com/assets/1396/474245/2323cc7a-b763-11e2-9b14-cd0daf756f6d.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

- [ ] Implement the traffic light logic, bind it up
- [ ] Make the button look more button-like
- [ ] Add a VS command for invoking GH4W (i.e. as if you hit the button)
- [ ] On initial installation, make Ctrl-S invoke above command, since SaveAllTheTime basically makes Save and Save All unnecessary
- [ ] On Uninstall, revert ^^