Adds Julia-style latex->unicode tab completion

This makes it much easier to use unicode identifiers in Python 3. This is achieved by converting latex symbols, such as `\alpha` to its unicode equivalent. This works without any modifications to the frontend. All I have done is a bit of new completer logic to the Python kernel.

Questions:

* Where should I put the notebook that autogenerates the map between latex-> unicode?

To do:

- [ ] Tests
- [x] Add logic to give a list of possible latex symbols
Yep, I think these are subtly different.

The reason I autogenerated the list from that of Julia is that I didn't want us to get into the horrible mess of deciding what characters to include and what exactly they are. Also, that way, as Julia iterates on their list, we can simple update ours easily.