[MRG+1] Learning curve: Add an option to randomly choose indices for different...

[MRG+1] Learning curve: Add an option to randomly choose indices for different training sizes (#7506)

* Chooses randomly the indices for different training sizes

* Bring back deleted line

* Rewrote the description of 'shuffle' attribute

* use random.sample instead of np.random.choice

* replace tabs with spaces

* merge to master

* Added shuffle in model-selection's learning_curve method

* Added shuffle for incremental learning + addressed Joel's comment

* Shorten long lines

* Add 2 blank spaces between test cases

* Addressed Joel's review comments

* Added 2 blank lines between methods

* Added non regression test for learning_curve with shuffle

* Fixed indentions

* Fixed space issues

* Modified test cases + small code improvements

* Fix some style issues

* Addressed Joel's comments - removed _shuffle_train_indices, more test cases and added new entry under 0.19/enhancements

* Added some modifications in whats_new.rst