added apps/notebooks/nbstripout 

This file, from https://gist.github.com/minrk/6176788, makes it easy to strip the output from a .ipynb file on disk without changing the live notebook.  This is useful when editing a notebook if you want to do a "git diff" or commit changes without affecting the output displayed in the notebook.  (You can of course clear all output from the notebook menu and save, but then you have to rerun all cells to get back to where you were).  

This also strips out the line
```
    "signature": "sha..."
```
that changes each time the notebook is used even if the input doesn't change.  

To use:

```
    $CLAW/apps/notebooks/nbstripout filename.ipynb
```

It should be possible to use this as a git hook and/or git filter too, according to https://gist.github.com/minrk/6176788, but I haven't yet implemented that.