Implement _BaseComposition

`sklearn.utils.metaestimators._BaseComposition` is an abstract class for parameter handling, which helps both [Pipeline](https://github.com/scikit-learn/scikit-learn/blob/7813f7efb/sklearn/pipeline.py#L29) and [FeatureUnion](https://github.com/scikit-learn/scikit-learn/blob/7813f7efb/sklearn/pipeline.py#L739) .