Cython Revisions

- [x] Revise discussion of what Cython is and what it us used for
- [x] Update description of how to use the Cython magic in IPython
- [ ] Include example of compilation with a `setup.py` script
- [ ] Remove discussion of fused types
- [ ] Show how to use the annotate option to analyse Cython code.
- [ ] Add warning about integer overflow
- [ ] Remove all discussion of the old NumPy syntax for arrays
- [ ] Remove discussion of how to use Memoryviews simultaneously with NumPy arrays
- [ ] Show how to use NumPy's gufuncs for arithmetic operations on memoryviews.