Execute ipynb and write a nbconverted html...

The idea here is to test a simple ipynb to see if it is rendering OK when it is nbconverted, and then if it would be nbviewer-ed OK.
I am using a custom script to execute and nbconvert the notebook, in the future we could change this for a `IPython.nbconvert` preprocessor currently in PR state, but not available until IPython 3.0 
yes, it could be... I just did it in this way thinking about testing all the ipynb in the future... when we have a proper event to catch the plot render end... in fact this discussion belongs to the other PR #709, which this one is based on...