Update of the IPython notebook plugin to support the upcoming 1.0 version of IPython.

OK, I have updated the plugin for blogging with the IPython notebook.
With the upcoming release of IPython 1.0, the nbconvert tools where merged into master, so I can use the IPython machinery for do the conversion instead of use a custom nbconvert library.

With this updated, the users only need to download the specific jinja-site-ipython theme and adjust their conf file in the way it is explained in the README.

I have also PR the theme to the nikola-themes repository to make it available and installable through Nikola.
Waiting for your comments.

Abrazo.
Damián