New GitHub Branding Slide Deck Theme

Cleans up the slide deck theme (Cobalt) and improves contrast for projector-intended showcasing.

Colors are from Primer and stay within branding colors. 

New ideas for bubble/sticky-note callout still a WIP.

@github/training-team discussion still necessary and further refinements at this initial PR before merging in.

![screen shot 2013-04-30 at 3 39 58 pm](https://f.cloud.github.com/assets/352082/446744/ecae4bae-b1de-11e2-9f1c-3314ad52ee70.png)
![screen shot 2013-04-30 at 3 40 21 pm](https://f.cloud.github.com/assets/352082/446745/ee624518-b1de-11e2-9d3f-36fb01264289.png)
![screen shot 2013-04-30 at 3 40 36 pm](https://f.cloud.github.com/assets/352082/446746/ef9e2370-b1de-11e2-9ee7-82745d89adf7.png)