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
Good point, @rkern, but it sounds like @takluyver's suggestion of using `str.isidentifier` is actually the cleanest solution, no?