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
Shouldn't this only be done for Python 3? The codemirror mode does include the Python 2/3 version.