updated docs to fix formatting errors
Merge pull request #2163 from ianozsvald/fix_plot_forest_iris_docs

Updated docs to fix formatting errors
PY3 use urllib2 or urllib.request, based on Py2/3

Also fixes #2166.
DOC let OneHotEncoder, DictVectorizer and FeatureHasher refer to each other

Judging by ML and SO questions, users have a hard time finding the right
vectorizers and don't know how to trade off the benefits of these three
classes.
DOC correct class_weight description for LogisticRegression
BUG: use correct algorithm for callable metric
TST: set random state in callable_metric test
Style
More readable condition and more precise error message