Fix IPython NB embed with custom width & height

@theengineear @chriddyp 

Custom iframe width & height are back !

![image](https://cloud.githubusercontent.com/assets/6675409/5055265/e355d624-6c2e-11e4-971a-5ef11193f898.png)


Related Asana tasks [here](https://app.asana.com/0/11724913287040/19683595083832) and [here](https://app.asana.com/0/14918644452133/19591876078728)