Add prototype for validation curve
Add documentation and tests
Improve test coverage

... to 99%
Improve interface and documentation
Mocks inherit from BaseEstimator
Correct docstring
Add narrative documentation
Improve documentation

* add images
* add link to API doc
* change order
Fix link to validation curve
Add example with polynomial regression
Correct and improve documentation
Improve phrasing
Rephrase sentence
Simplify first part of the documentation
Fix typo
Return scores of all folds

It is possible to plot confidence intervals with fill_between
Fix documentation
Improve test coverage
Matching colors
Merge pull request #2765 from AlexanderFabisch/validation_curves

[MRG] Validation curves