Fix indent and center

Some weeks ago they were merged two PR changing some UI things: #4154 and #4567.
These PR bring some changes in the centring of equations and indentation of non code cells.
This PR takes into account this changes and translate it to `nbconvert`.

Here is the screenshot of current `html_full` from `nbconvert`:

![ui1](https://f.cloud.github.com/assets/1640669/1763553/634b7ca4-6711-11e3-8dad-c26a155efc18.png)

And this one is the new look proposed by this PR, which follow the last changes in the notebook appearance:

![ui2](https://f.cloud.github.com/assets/1640669/1763558/6980b9a4-6711-11e3-91a2-d485e42ad326.png)

I have also regenerated the `null.tplx` because it was not updated (missing a lower() in the raw cell block).

Now, I think is ready for review.