Replace required Cython dependency with optional Numba

[sethaxen]

Having Cython as a required dependency puts constraints on this package. For example, we have to build wheels for multiple systems. We're only using it to accelerate a single function, so this PR replaces instead uses Numba if available.

[coveralls]

Coverage increased (+0.8%) to 59.041% when pulling ddf06cb on numba into eac3c80 on master.