Fixed issue 3815. Discrete AdaBoostClassifier now fails early if the base classifier if worse than random.
TST stronger non-regression test for #3815

Also fixed the testing imports.
Merge branch 'pr/3837'

Fixes gh-3815.
FIX make assert_raises_regex backport for 2.6 consistent with 2.7+

Use regex.search instead of regex.match.

Furthermore, silence the deprecation warning from Python 3.4 as
assert_raises_regexp is now deprecated in favor of the shorter name:
assert_raises_regex.
Merge pull request #3861 from ogrisel/fix-python-2.6-test

[MRG] FIX make assert_raises_regex backport for 2.6 consistent with 2.7+
gen_rst informs file with missing documentation