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 this changes into account and translate it to `nbconvert`.
I have modified the `null.tpl` trying to follow the logic of the live notebook (and the `nbconvert` itself)...

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)

A problem I see with the proposed implementation is the code duplication when I define all the input_groups... so if anybody have better ideas to deal with it, I am open to hear... and also any other critics :wink: