Added
- First alpha of using Rust. `optimalK` now takes `"rust"` as an argument for the parallel_backend
This is the fastest backend, where joblib w/ scipy had ~7.5s in benchmark, Rust is ~4.5s
*Pre-built wheels for Linux & OSX for Python 3.5 & 3.6. Windows users can install but will need
Rust nightly in order to compile.*