Implement OrdinalEncoder

[VolodymyrOrlov]

Implement OrdinalEncoder, make logic similar to Scikit Learn's

The new encoder should belong to new module preprocessing and produce results on par with Scikit Learn

[gaxler]

As of now we limit categorical encoding to round RealNumbers, so for now OrdinalEncoder will be an identity function.

I'll adjust the SeriesOneHotEncoder so it will be able to make an ordinal encoding as well (with a more general CategoryMapper object). But won't implement any Ordinal transformers for now.