Use Cython instead of Numba

This pull request replaces all of the numba code with Cython, as well as transitioning some of the pure-Python code (e.g. in `gp.py`) to Cython as well.

Changes include:
* Using Miniconda to install numpy and scipy on Travis
* Changing the `src` directory to be the `gp` package
* Creating the `ext` package, which contains all the Cython code
* More tests