Add right margin to HTML tag to make room for inspector pane

I'm wondering if this functionality was considered and then rejected depending on the possible mutilation of the user's stylesheet, but I think just modifying `margin-right` of the `<html>` tag is not too intrusive. An example:

Before:
![image](https://cloud.githubusercontent.com/assets/38674/3185253/fe43824c-ec90-11e3-9ab4-42c90b0766e9.png)

After:
![image](https://cloud.githubusercontent.com/assets/38674/3185255/06c97642-ec91-11e3-9ed4-579697670cd7.png)

Really helps with smaller displays!