Cleanup of initial notebook as the interface opens up.

I deleted the help message. The rest is up to Toni

Mostly done. But I'd still like to get rid of cell 0 altogether, and start the user-visible counter at 1. IPython trunk does this by sticking an empty input at 0 and never displaying it. I don't know the code enough yet to fix it completely.

The matplotlib code is still in, just that it's hidden. Later we'll make it optional, once we have proper profiles (like ipython trunk).

I think that the matplotlib code must be a part of the notebook. If it was a part of a profile then we would have to include a reference to the profile in the notebook, and that would make the notebook unportable. The profile cannot be set by command option or a preference to nbshell. Imagine , for example, that the user wants to open a Matplotlib notebook and after that a Gnuplot notebook (when we add Gnuplot support). If we don't keep the data inside the notebooks we would have to restart nbshell or change some preferences to do that.

Ideally I think that there should be a hidden initialization cell in the notebook which will perform the necessary initializations every time a notebook is open. This is the role of the zero cell. It should not be a part of the sheet. The method Notebook.default_sheet() in nbdoc which makes the new sheet shows this cell. I will change it so the zero cell is hidden.