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
For now, I would use [pairwise_distances](http://scikit-learn.org/stable/modules/generated/sklearn.metrics.pairwise_distances.html#sklearn.metrics.pairwise_distances). It will be faster