Fix the issue #3745, the code book generation for OutputCodeClassifier

+ Change the process of generating output code for OutputCodeClassifier. The process is to draw subsets of the exhaustive code book, see [1], multiple times and pick the one that give largest hamming distances between classes.
+ Change the default value of code_size from 1.5 to 1. 1.5 is problematic. For example when n_classes = 3, the size of exhaustive code book is 3, so 1.5 for code_size is not possible.
+ Add test case.
+ Update the document.

[1] Thomas G. Dietterich, Ghulum Bakiri. Solving Multiclass Learning Problems via Error-Correcting Output Codes
By the way, it would be nice to have list of strategy displayed as https://github.com/MechCoder/scikit-learn/blob/manhattan_metric/sklearn/neighbors/classification.py#L46