Added support to generate a naive ipynb and fixed README.

I made some changes to make more easy to integrate the work with Nikola and IPython.
Now, when you call:

$nikola new_post -f ipynb

Nikola will write two files: a meta file and a **naive** ipynb with the proper title. This one can be modified by IPython just starting the notebook in the post folder.

Abrazo.

Damián.