Adding new Tributary screenshots for index.html

It seemed like it was time to update the screenshots on the front page, as they don't really reflect what Tributary looks like anymore. The third screenshot was the hardest to figure out, as we don't have share buttons anymore. I think the closest 'social/sharing' aspect I find of it is creeping through everyone's gists to see what they're working on, so I put a screenshot of that.

I think in future it might be useful to maybe change it to 1 screenshot and 2 more icon/designy images to accompany the text underneath.

![Screen Shot 2013-01-04 at 1 38 32 PM](https://f.cloud.github.com/assets/530073/43864/433dd738-569e-11e2-842b-c3b065545d88.png)