Feature-engine

**Big release**

The new release adds all of the following:

- New transformer: datetime
- New transformer: selection by PSI
- New transformer: match variables
- New documentation layout: pydata template
- New documentation organisation: api and user guide
- Sponsorship
- Paper and citation

And a lot more expended functionality of current classes.

Fixes bug in one hot encoder

* Migrates repo to own organisation github.com/feature-engine
* adds new transformer (logcp)
* expands transformers with inverse_transform
* expands correlation selectors by allowing callables in the corr method

- enforce compatibility with Scikit-learn through check_estimator
- new transformer for cyclical features
- add categorical support for variables of type category
- add option to perform categorical transformation to numerical variables
- feature selectors support all cross-validation schemes

In this release we add:
- a new transformer for imputation
- a new transformer to create features
- new jupyter notebooks with demos
- linkd to Kaggle kernels featuring Feature-engine
- fix a number of typos in the documentation
-fix few bugs in the select by target mean transformer.