Updated update icons and tweaked update dialog typography.

Since we're going to use a green icon to show that there extension updates available we should also make the Brackets update icon green for consistency. I've also tweaked the typography in the update dialog.

To test this go to Help > Check for Updates.

![screen shot 2014-03-29 at 2 30 45 pm](https://cloud.githubusercontent.com/assets/1495261/2559737/e1bf6618-b789-11e3-8f1e-4774f4e9f6fe.png)

![screen shot 2014-03-29 at 2 15 00 pm](https://cloud.githubusercontent.com/assets/1495261/2559735/dc6db250-b789-11e3-91fb-ff247319ca44.png)