Allow for multiclass classification

[iKintosh]

Hi,
I noticed that this binding works only with binary classification. I added small change to allow for multiclass classification.
Ideally it should put vec! inside vec!, but it requires changes in a couple of places as well.

[jlloh]

Hello, thank you for your contribution!

1. Would it be possible to add some unit basic tests for multiclass classifications?
2. We should probably bump the package version in Cargo.toml ?
3. Perhaps also worth starting a changelog.md file, I can add one and you can rebase from master and add your changes to the changelog