Notebook save hooks

Post save hooks for notebooks, uses the notebook metadata to define a postsavehook. replaces the '--script' useage in the notebook server.

Workflow:
* Activate the extention via %reload_ext IPython.frontend.html.notebook.examples.saveasscript 
* choose 'save as .py: enable'
* save notebook :-)

To check which cells will be written to the .py file choose 'Cell Toolbar: auto export hin' and change the setting for each cell.

Closes: #3295