Ensure urllib.quote input is `str`, not `unicode` to avoid choking on non-ASCII paths

Love the workflow. Awesome idea!

Unfortunately, it chokes on non-ASCII data, as `urllib.quote()` (very correctly) doesn't accept non-ASCII Unicode and [Alfred-Workflow](https://github.com/deanishe/alfred-workflow) (arguably correctly) gives you Unicode arguments.

I've added the necessary checks to ensure that `urllib.quote()` only gets passed `str` objects, not `unicode`.

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If I might make a further suggestion, you could use `mdfind -name '.ipynb'` via `subprocess` to get a list of all iPython notebooks on the system extremely quickly from Spotlight's index.

I'm not yet that familiar with iPython (I only installed it yesterday, just for the notebooks), but I think that might possibly be a better way to get a list of iPython notebooks than manually scanning the whole file tree.

I have no idea if iPython's server can open notebooks outside its root directory, but if you can get that path from iPython, you can use `mdfind -onlyin '/ipython/root/dir -name '.ipynb'` to restrict the search to that location.

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Finally, I'm always unjustifiably happy to see [Alfred-Workflow](https://github.com/deanishe/alfred-workflow) being used, and if you have any feedback, I'd love to hear it.

Would it be okay if I added your workflow to the list of workflows using Alfred-Workflow?

It's not very long at the moment, and most of them were written by me :disappointed: